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213 Cards in this Set
- Front
- Back
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Definition of epidemiology
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The study of distribution and determinants of health-related states or events in specified populations and the application of this study to the control of health problems
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Epidemiology is concerned with...
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frequency and pattern of health events in a popluation
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Frequency
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not only the number of health events such as the number of cases of diabetes in a population, but also the relationship of that number to the size of the poulation
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Pattern
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refers to the occurence of health related events by time, place and person
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Determinants
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The causes and other factors that influence the occurence of disease and other health related events
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2 type of measure of disease frequency:
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PREVALENCE
INCIDENCE 1. Cumulative Incidence (CI) 2. Incidence rate (IR) |
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2 type of measurement of incidence:
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Cumulative Incidence (CI)
Incidence rate (IR): |
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Active immunity
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when the body produces antibodies when you are infected or by vaccine
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Acute (Active) Outbreak
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can be deductive or inductive
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Case control
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Selection on basis of DISEASE status.
Investigator go back to determine if they had/had not exposure. |
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Chance
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a. Result due to random sampling variability
b. To measure the effect of chance is by doing a test of statistical significance. |
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Cohort
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Selection on basis of EXPOSURE status.
Investigator start with exposure status and track over time to see what happens to disease. |
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Cumulative Incidence (CI):
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the portion of people who become disease over specific time.
CI is an estimate for the risk (probability) that a person will develop disease in a time frame. |
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Cumulative Incidence CI =
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CI = # of new cases developing in specified time / Total population @ risk
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deductive
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Looking back to see what caused disease
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Edward Jenner
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smallpox vaccine
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Endemic
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Habitual present of disease in an area or constant presence and normally expected
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Epidemic
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at a grand scale (in group- increase in # of what is normally expected) – excess of normal expectancy
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Hippocrates
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• 1st epidemiology
• rational vs. supernatural explanation • Disease not only affects individuals but also pop. • Differentiated between endemic and epidemic • Associates between environment and diseases |
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INCIDENCE
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development of disease who are at risk over a period of time.
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Incidence rate (IR)
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the rate at which new cases of disease occurs in a population at risk for disease.
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Incidence rate (IR)=
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IR = # of new cases developing over study period / Total person-time of observation
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inductive
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Looking ahead to see what the disease can cause.
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Intervention Study
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Comparison of an outcome (e.g. morbidity or mortality) between two groups of people deliberately subjected to different dietary or drug regimes.
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John Snow
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cholera --> contaminated water
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Null hypothesis (Ho) =
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NO association
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Pandemic
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international borders (worldwide epidemic)
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Passive immunity
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when you acquire immunity by:
a. Injection of serum b. Placental transfer c. Breast feeding |
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Primary prevention:
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a. Prevent the development of disease
b. Does not have the disease in question (yet). i. Therefore, you can immunize that person. c. If disease is environmentally induced: we can prevent that person’s exposure to the environmental factor |
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Prospective cohort study
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study in which disease has not yet occurred at initiation of study.
Begin at midpoint and track to FUTURE to see what happens to disease upon exposure |
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Retrospective Cohort study
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Disease occurred in the past prior to initiation of study.
Start w/ exposure status and track BACK to examine effect of exposure or non-exposure on disease |
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Secondary prevention:
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a. Focus on early detection and fast treatment of disease- the purpose is to cure or slow the progression of disease.
b. Use test to screen: i. Breast examination ii. Mammogram iii. Eye test iv. Blood test v. Etc. |
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Steps to define acute outbreak:
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1. Define the epidemic:
a. Define the # of cases (numerator). b. Define the population at risk (denominator). 2. Examine distribution of cases by: Time, place and time-place interactions. 3. Examine combinations of relevant variables. 4. Develop hypothesis. 5. Test hypothesis. 6. Control measures |
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Tertiary prevention:
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a. Therapeutic and rehabilitative when disease is established.
i. Change lifestyle with rehab or hospital program, or medication |
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To determine if study is valid:
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1. Chance
2. Bias 3. Confounding |
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What does the prevalence Data measure?
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It measures the disease frequency
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Can relative risk (RR) be calculated with case-control study?
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No, but RR is approximated by odds ratio (OR) = ad/bc
so calculated OR & interpret as relative risk |
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In ecological model, what factors affect human disease ?
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host characteristics
types of agents environmental factors |
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Relative Risk (RR)
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indicate a measure of the strength of the association between an exposure and a disease.
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Relative risk formula:
RR = ? |
RR = CI(Exposed)/CI(not exposed)
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What does the null hypothesis mean?
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It means that the independent variable does not have an effect.
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What is Relative Risk (RR)
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Tells those who are exposed have X times the risk of developing the outcome than the nonexposed
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what is the difference between case control and retrospective cohort?
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case control: INVESTIGATOR goes back see when or not exposed
retrospective cohort: start w/ EXPOSED and go back when or not exposed. |
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What is the purpose of a control group?
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allow the researcher to determine the influence of external factors that are not suppose to influence the results of the study.
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What method can you use to control variables in a study ?
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1.Randomly selecting the subject
2.remove the variable from the study 3.Match subjects in comparison groups by criteria 4.randomly assign subjects to either control or experimental group |
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Define a case-control study and list some pros/cons
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The study is usually conducted by small research teams and involves identifying the outcome first and then choosing different risk factors to explore. Groups of cases are chosen by using medical records or interviews. Controls are then selected.
pros: cheap cons: no indication of absolute risk, confounding |
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Define a cohort study and list some pros/cons
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Compares 2 groups; One group is exposed to another group that is unexposed or has a different level of exposure.
it is followed over time to determine differences in outcome. pros: reduces some forms of bias and controls confounders cons: cannot distinguish causality from correlation, expernsive, lengthy, and possible bias due to multiple measures. |
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Define a cross sectional study and list some pros/cons
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A cross sectional study is used to assess acute or chronic conditions in a population. It determines prevalence.
A disease and exposure status are examined at the same time. pros: quick, cheap, allows collecting of multiple covarients cons: cannot assess causality, confounding, and bias |
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What are the 2 triads of analysis
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person/place/time
agent/host/environment |
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What are the 4 components of a case definition?
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1)Clinical information on the disease
2)Characteristics of the people affected 3)Information about location or place 4)Time during which an outbreak occured |
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What are the 5 components of Public Health Surveillance?
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1)Ongoing collection of data
2)Interpretation of data 3)Analysis 4)Dissemination of data (spread the word) 5)Action linked to the health/risk factor |
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Causal Pies
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model used for non-infectious diseases, such as diabetes
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Individual factors of a causal pie
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component causes
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Completes causal pie
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sufficient cause
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Component that appears in every causal pie/pathway
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necessary cause
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Natural history of disease
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Refers to the progression of disease progress in an individual over time, in the absence of treatment
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Incubation period (infectious disease) and latency period (chronic disease)
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Time when disease process has been triggered and pathological changes occur without the individual being aware of them.
Person is asymptomatic Can last minutes or even decades |
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Spectrum of disease
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The way in which a disease manifests itself, can be mild or severe. It can also never become clinically apparent
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Infectivity
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Proportion of persons who become infected
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Pathogenicity
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Proportion of infected individuals who develop clinically apparent disease
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Virulence
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Proportion of clinically apparent cases that are severe or fatal
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Carriers
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Persons who are infectious but have subclinical disease. Are frequently persons with incubating disease or inapparent infection.
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When does transmission occur?
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When the agent leaves its' reservoir or host through a portal of exit, is conveyed by some mode of transmission and enters through an appropriate portal of entry to infect a suceptible host.
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Reservoir
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The habitat in which the agent normally lives, grows and multiplies. Can be a human, animal or environment)
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Human reservoirs
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many common infectious diseases have human reservoirs (measles, mumps, smallpox). Humans may or may not show effects of illness.
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Asymptomatic/healthy carriers/passive carriers
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never experience symptoms
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Incubatory carriers
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Those who can transmit agent during incubation period before clinical illness begins
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Convalescent carriers
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Those who have recovered from illness but are still capable of transmitting to others
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Chronic carriers
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Those who continue to harbor a pathogen for months or even years after intital infection. (Example: Hep B)
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Animal reservoirs
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Animal to animal transmission, with humans as incidental hosts
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Zoonosis
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an infectious disease that is transmissible under natural conditions from vertebrate animals to humans (West Nile, monkey pox, rabies)
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Environmental reservoirs
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Plant/soil/water. Many fungal agents live in soil.
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Portal of exit
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The path by which a pathogen leaves the host, usually corresponds to the site where pathogen is localized (Example: urine/respiratory tract/skin)
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Direct Transmission
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An infectious agent is transferred from a reservoir to a suceptible host by direct contact or droplet spread
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Direct contact
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Skin to skin, kissing and sexual contact. Can also refer to contact with plants or soil.
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Droplet spread
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refers to spray with relatively large, short range aerosols produced by sneezing, coughing, or even talking. Transmission is by direct spray over a few feet before the droplets fall to the ground. Example: pertusis and meningococcal infections
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Indirect transmission
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Transfer of an infectious agent from a reservoir to a host by suspended air particles, inanimate objects, or animal intermediaries
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Airborne
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When infectious agents are carried by dust or droplet nuclei in the air (These droplets can be suspended in the air for long periods of time and can be blown over great distances becasue they are so small) Example: Measles
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Vehicles
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Food, water, blood, fomites (bedding/scalpels etc.) Example: Hep A in water, or botulinum toxin in food
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Vectors
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Mosqutoes, fleas, ticks...Can carry through purely mechanical means, or support growth or changes in the agent. Mechanical example: fleas carring plague agent in their gut. Biologic transmission example: malaria ungergoes tranformation in intermediate host before it can be transmitted to humans.
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Portal of entry
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The manner in which a pathogen enters a suceptible host. Oftentimes, infectious agents use the same portal to enter as they did to exit. Example: influenza exits and enters through respiratory tract. Can also enter through skin, fecal-oral, mucous membranes, or blood.
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Host
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Final link in the chain of infection. Suceptibility depends on genetic or constitutional factors, specific immunity or nonspecific factors. Factors that may increse suceptibiltity include malnutrition, alcoholism anddisease that impairs immune response.
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Interventions
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Directed at controlling or eliminating agent at source of transmission, protecting portals of entry, and increasing host defenses
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Herd immunity
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A concept that suggests if a high enough proportion of individuals in a population are resistant to an agent, then the few who are not will be protected by the resistant majority becasue the agent is unlikely to find the suceptible individuals
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Endemic level
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The amount of disease usually present in a community, not necessarily the desired level, but the observed level.
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Sporadic
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Refers to disease that occurs infrequently and irregularly
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Hyperendemic
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Persistent high levels if disease occurence
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Epidemic
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An increase, often sudden, in number of cases of a disease above what is expected
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Outbreak
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Same definition as epidemic, but is often used for a more limited geographic area
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Cluster
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a aggregation of cases grouped in place and time that are suspected to be greater than the number expected, even if the expected number is not known.
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Pandemeic
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Refers to an epidemic that has spread over several countries or continents
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Epidemics result from:
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1) A recent increase in amount or virulence of aggent
2) The recent introduction of the agent into a setting where it had not been before 3) An enhanced mode of transmission so that more suceptible persons are exposed 4) A change in the suceptibility of the host response to the agent and/or 5) Factors that increase host exposure or involve introductionthrough new portals of entry. |
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Common source outbreak
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One in which a group of persons are all exposedto an infectious agent or toxin from the same source
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Point source outbreak
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If the group is exposed over a relatively brief period, so that everyone who becomes ill does so within one incubation period, then the common source outbreak is further classified as this. This epidemic curve would have a steep upslope and a more gradual downslope (aka "log-normal distribution") Example: Leukemia after Hiroshima and Hep A after eating green onions in PA.
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Continuous common source outbreak
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Case patients are exposed over a period of days, weeks or longer. The range of exposures will tend to flatten and widen the peaks of the epidemic curve.
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Intermittent common source outbreak
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has a pattern refelective of the intermittent nature of the exposure, lasts more than 1 incubation period, and a common source can be identified
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Propagated outbreak
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Results from transmission from one person to another, can be direct contact (syphillis), vehicleborne (HIV though sharing of needles), or vectorborne (yellow fever via mosquito bite). These cases occur over more than one incubation period. The epidemic usualy wanes after a few generations
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Mixed epidemics
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Have features of both common source epidemics and propagated epidemics. The pattern of a common source outbreak followed by secondary person to person spread is not uncommon.
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Epidemics that are neither common source nor propagated from person to person
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Example: Lyme disease and West Nile in NYC
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Ratio
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Relative magnitude of 2 quantities or comparison of any two values. Calculate by dividing one interval ratio scale by another. The numberator and denominator need not be related. Often expressed as the result "to one" or written as :1
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Ratios
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Used as both a descriptive and analytic tools. Descriptive example: male to female participants in a study. Analytic example: occurence of illness or injury between two groups. THE VALUE OF BOTH NUMBERATOR AND DENOMINATOR ARE DIVIDED BY THE VALUE OF ONE OR THE OTHER SO THAT EITHER EQUALS ONE.
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Death to Case Ratio
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Number of deaths attributed to a particular disease during a specified period diveided by the number of new cases identified in the same period. Measures the severity of illness.
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Proportion
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A comparison of a part to a whole. The numerator IS included in the denominator, and is often expressed in percentage. 10n is usually 100 (or n=2). Most often used as a descriptive measure, but can be used to describe abount of disease that can be attributed to a particular exposure
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How to convert proportion to ratio
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Subtract numerator from denominator or vice versa(depending on the ratio you need)and divide
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Proportionate mortality
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Proportion of deaths in a specified population during a period of time that are attributable to different causes. Each cause id expressed as a persentage of all deaths, and the sum of the causes adds up to 100%
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Rate
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measure of frequency with which the event occurs in a define population over a specified period of time. Rates are a measurement of risk
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Incidence rate
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The speed at which a disease occurs over a period of time
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Attack rate (same as incidence proportion)
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Proportion of population that develops illness during an outbreak
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Prevalence rate
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Proportion of poulation that has a health condition at a specific point in time
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Case fatality rate
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Proportion of persons with the disease who die from it
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Morbidity
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any departure, subjective or objective, from a state of physiological or psychological well being. It encompases disease, injuray and disability
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Incidence proportion
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Numerator: Number of new cases during a specified time interval
Denominator: Population at start time of interval |
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Secondary attack rate
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Numerator: Number of new cases among contacts
Denominator: Total number of contacts |
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Incidence rate
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Numerator: Number of new cases during a specified time interval
Denominator: Summed person-years of observation or everage population during time interval |
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Point prevalence
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Numerator: Number of current cases (new and preexisting) at a specified point in time
Denominator: Population at the same specified point in time |
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Period prevalence
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Numerator: Number of current cases (new and preexisting) at a specified point in time
Denominator: Average or mid-interval population |
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Incidence
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Occurence of new cases or injury in a population over a specified period of time
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Synonyms for incidence proportion
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Attack rate
Risk Probability of developing disease Cumulative incidence |
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Definition of incidence proportion
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The proportion of an initially disease free population that develops disease, becomes injured or dies during a specified period of time.
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Properties and uses of incidence proportion
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Measure of risk or the probability of developing the disease. As a measure of incidence, it only includes NEW cases in the numerator. All the persons in the numerator are included in the denominator, so it is also a proportion.
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Attack rate
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Risk of getting the disease during a specified period
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Overall attack rate
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Total number of new cases divided by the population
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Food-specific attack rate
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Number of persons who ate a specific food and became ill divided by the total number of persons who ate that food.
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Secondary attack rate
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Sometimes calculated to document the difference between community transmission of illness versus transmission of illness in a household or other closed poulation. Calculated as Number of cases among contacts of primary cases divided by number of contacts times 10n. 10n is usually 100%. See example on p3-12
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Incidence rate
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The ratio of the number of cases to the total time the population is at risk for the disease. The demoninator is the sum of the time each person was observed, totalled for all persons. This represents the total time the poulation was at risk for and being watched for the disease
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What is the term used for a person followed for 5 years without developing the disease?
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5 person years of follow up
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What is the contribution to the denominator if a person is lost from the study before the next year follow up?
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1/2 year is added to the denominator. (See p. 3-13). One 1/2 year is also used for the person who develops the disease at teh next year follow up. This is based on an assumption that the person was disease free for 1/2 year.
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Incidence rate
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describes how quickly disease occurs in a population, is based on person-time, so it has some advantages over incidence proportion. It accomodates for persons leaving the study or die during the study. It also allows for enrollees to enter the study at sifferent times.
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Person time drawback
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Assumes the probability of disease during the study period is constant, so 10 persons followed for one year equals 1 person followed for 10 years. Becasue of age related factors, this assumptions is often not valid.
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Person years can also be stated as...
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persons per year
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Prevalence/prevalence rate
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proportion of persons in a population who have a particular disease or attribute at/over a specified period of time.
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Point prevalence
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prevalence measured at a particular point in time
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Period prevalence
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Prevalence measured over an interval of time.
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Method for calculating prevalence of disease
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All new and pre-existing cases during a given time period divided by population during the same time period multiplied by 10n
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Method for calculating prevalence of an attribute
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Persons having a particular attribute during a given period of time divided by population during same time period multiplied by 10n
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Prevalence
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proportion of persons who HAVE a condition at a particular time period
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Incidence
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Proportion of persons who DEVELOP a condition at a particular time period
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Numerator of incidence
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NEW cases that occured during a given time period
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Numerator of prevalence
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ALL cases present during a given time period, regardless of when the illness began
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What is used for chronic illness, incidence or prevalence?
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Prevalence, because they have a long duration and onset is often difficult to pinpoint.
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Mortality rate
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Measure of the frequency of occurence of death in a define population during a specified interval
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Formula for mortality of a defined population
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deaths occuring diring a given time period divided by the size of the population among which the deaths occured multiplied by 10n
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Crude death rate
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Numerator: Total number of deaths during a given time interval
Denominator: Mid-interval population 10n=1,000 or 100,000 |
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Cause-specific death rate
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Numerator: Number of deaths assigned to a specific cause during a given time interval
Denominator: Mid-interval population 10n=100,000 |
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Proportionate mortality
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Numerator: Number of deaths assigned to a specific cause during a given time interval
Denominator: Total number of deaths from all causes during the same time interval 10n=100 or 1,000 |
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Death to Case Ratio
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Numerator: Number of deaths assigned to a specific cause during a given time interval
Denominator: Number of new cases of same disease reported during the same time interval 10n=100 |
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Neonatal mortality rate
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Numerator: Number of deaths among children <28 days of age during a given time interval
Denominator: Number of live births during the same time interval 10n=1,000 |
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Postneonatal mortality rate
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Numerator: Number of deaths among children 28-364 days of age during a given time interval
Denominator: Number of live births during the same time interval 10n=1,000 |
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Infant mortality rate
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Numerator: Number of deaths among children <1 year of age during a given time interval
Denominator: Number of live births during the same time interval 10n=1,000 |
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Maternal mortality rate
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Numerator: Number of deaths assigned to pregnancy-related causes during a given time interval
Denominator: Number of live births during the same time interval 10n=100,000 |
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Crude mortality rate
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Mortality rate for all causes of death for a poulation
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Cause-specific mortality rate
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Mortality rate from a specified cause for a population
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Age specific mortality rate
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mortality rate limited to a particular age group
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Infant mortality rate
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Widely used and reflects the health of the mother and the infant during pregnancy and the year after
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Is the infant mortality rate a ratio or proportion?
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It is a ratio, because some of the deaths in the numerator were children born in the previous year.
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Neonatal mortality rate
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covers period of birth up to but not including 28 days.
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Postneonatal mortality rate
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Number of deaths from children 28 days up to but not including age 1
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Maternal mortality rate
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ratio used to associate mortality with pregnancy
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Sex specific mortality rate
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the mortality rate among males or females
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Race specific Mortaility rate
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Mortality rate related to a specified racial group
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Combinations of specific mortality rates
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Mortality rates can be further stratified by combination of cause/age/sex/race.
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Age adjusted mortality rates
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A mortality rate statistically modified to eliminate the effect of different age distributions in the different populations
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Death to case ratio
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Number of deaths attributed to a particular disease during a specified time period divided by the number of new cases that disease identified during the same time period.
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Case fatality rate (aka case fatality ratio)
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Proportion of persons with a particular condition (cases) who die from that condition. It is a proportion, so the numerator is restricted to deaths among people included in the denominator.
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Proportionate mortality
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Proportion of deaths in a specified population over a period of time attributable to different causes. Each case is represented in percentage, and total percentage must equal 100%.
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Proportionate mortality ratio (PMR)
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Used to compare deaths in a poulation of interest to the broader population. PMR greater than 1 indicates that a particular cause accounts for a greater proportion of deaths in the population of interest than you might expect.
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Healthy worker effect
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People who work more are more likely to be healthier than the population as a whole
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Years of potential life lost
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one measure of the impact of premature mortality on a population. YPLL is the sum of differences between a predetermined end point and the ages of death for those who died before that endpoint. Commoly used are 65 years and average life expectancy.
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How to calculate YPLL from a line listing
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1) Decide on end point
2) Exclude records of all persons who died at or after the end point 3) For each person who died before the end point, calculate YPLL by subtracting the age at death from the end point 4) Sum all YPLL's |
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How to calculate YPLL from a frequency
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1) Ensure age groups break at identified end point. Eliminate all age groups older than end point.
2) For each age group younger than the end point, identify the midpoint of the age group (youngest age+oldest age+1 divided by 2) 3) for each group younger than the end point, identify that age group's YPLL by substracting the midpoint from the endpoint. 4) Calculate age specific YPLL by multiplying the age group's YPLL by the number of persons in that group 5) Sum the age specifi YPLL's |
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YPLL rate
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Represents years of potential life lost per 1,000 population below the end point age. Calculated as YPLL divided by population under end point age multiplied by 10n.
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Natality Measures
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Population based maeasures of birth
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Crude Birth Rate
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Numerator: Number of live births during a specified time interval
Denominator: Mid interval poulation 10n=1000 |
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Crude fertility rate
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Numerator: Number of live births during a specified time interval
Denominator: Number f women ages 15-44 years at mid-interval 10n=1000 |
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Crude rate of natural increase
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Numerator: Number of live births minus number of deaths during a specified time interval
Denominator: Mid interval poulation 10n=1000 |
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Low-birth weight ratio
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Numerator: Number of live births <2,500 grams during a specified time interval
Denominator: Number of live births during the same time interval 10n=100 |
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Measure of association
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Quantifies the relationship between exposure and disease among two groups. Exposure is not only exposure to something like food or toxic waste, but is also inherest characteristics, activities or living conditions
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Examples of measures of association
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1) Risk ratio
2) Rate ratio 3) Odds ratio 4) Proportionate mortality ratio |
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Risk Ratio (aka relative risk or RR)
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compares the risk of a health event among one group with risk among another group. Divide risk in group 1 by risk in group 2
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A risk ratio of 1 indicates:
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identical risk in the two groups
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A risk ratio greater than 1 indicates:
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increased risk for the group in the numerator (usually the exposed group)
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A risk ratio less than 1 indicates:
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Decreased risk for the exposed group, indicating that perhaps exposure protects against disease occurrence
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Rate Ratio
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Compares incidence rates, person-time rates or mortality rates of 2 groups and is calculated by dividing the rate for group of primary interest by rate for comparison group.
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Rate ratio of 1.0:
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Indicates equal rates in the two groups
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Rate ratio greater than 1:
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Indicates increased risk for the group in the numerator
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Rate ratio of less than 1:
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Indicates decreased risk for the group in the numerator
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Odds Ratio
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Measure of association that quantifies the relationship between exposure with two categories and health outcome. Calculated as ad/bc
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Case control measure of choice:
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Odds ratio becasue in a case control, ther are persons with disease and without disease
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Attributable proportion (aka attributable risk percent)
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measure of the public health impact of a causative factor. Calculated by subtracting risk for unexposed group from risk for exposed group, dividing by risk for exposed group and multiplying by 100%
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Vaccine efficacy
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measure the proportionate reduction in cases among vaccinated persons. It is calculated by subtracting risk among vaccinated group from risk among unvaccinated group and dividing by risk among unvaccinated group or 1-risk ratio
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Frequency (Distribution)
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Number of events and their relationship to the size of the population
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Pattern (Distribution)
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The occurence of health related events by time, place and person
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Determinants
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Causes that influence the occurence of disease and other health related events
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Application
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Apply the knowledge gained to community based practice
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Public health surveillance
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The ongoing systematic collection, analysis, interpretation and dissemination of health data to help guide public health decision making and action
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Evaluation
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The process of determining, as systematically and objectively as possible, the relevance, effectiveness, efficiency and impact of activities with respect to established goals
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Effectiveness
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Refers to the ability of a program to produce the intended or expected results in the field. Differs from efficacy, which is the ability to produce results under ideal conditions
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Efficiency
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Ability of a program to producethe intended results with minimum expenditure of time and resources
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Linkages
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The other team members involved in public health settings. Examples of mechanisms for sustaining linkages are published information and informal networking
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An epidemiologist:
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Counts, divides and compares
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Case definition
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a set of standard criteria for classifying whether a person has a particular disease, syndrome or condition. There may be several sets of criteria. Example: suspected, probable or confirmed
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Sensitive case definition
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One that is broad or "loose" in hopes of capturing most or all of the true cases
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Strict case definition
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Specific infections must be confirmed, ensuring they truly have the disease
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Rate
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The number of cases divided by the size of the population per unit of time
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5 W's of descriptive epidemiology
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What, who, Where, When and Why/how
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Secular trends
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Long term trends
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Seasonality
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Disease occurence can be graphed by week or month over the course of a year. Example: flu
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Epidemic curve
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A graph used to show the time course of a disease outbreak or epidemic
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Incubation period
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Length of time between exposure and conset of symptoms
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Spot maps
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Generally used for clusters or outbreaks with a limited number of cases
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Person attributes include
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Age, sex, ethnicity/race, and socioeconomic status
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Socioeconomic status
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Frequency of many adverse health conditions increase with decreased socioeconomic status. This may reflect harmful exposures, lower resistance and less access to health care.
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Key feature of analytic epidemiology
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Comparison groups
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