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40 Cards in this Set

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Epidemiology
Study of distribution and determinants of health and disease in POPULATIONS
Primary prevention
prevent disease from occuring
Secondary prevention
early detection to change natural history
Tertiary prevention
reducing disability after treatment
Determinants of distribution
Infectious agents
Carcinogens, chemicals
Behaviors, habits, lifestyle
Occupational exposures
aging
genetic constitution
social/political factors
Epidemiologic triad includes?????
Host
Environment
Agent
Distribution:
Who
Where
When
How
Changes
Epidemic?
Higher than normal levels of disease
John Snow used what approach to epidemiology?
Ecologic and observation or retrospective cohort design
Koch's postulates
1. agent recovered from all diseased individuals
2. Agent grows in culture
3. Cultured organism causes new disease in susceptible
4. Agent recovered from new diseased animal
Revised causal criteria for disease
1.Temporal relationship
2.Strength of association (relative rate)
3. Biologic plausibility
4. Risk factor
5. Consistency: association is replicated by other investigators
6. Dose response
Herd immunity
1. Resistance to the spread of infectious disease in a group because susceptible members are few, making transmission from an infected member unlikely.
2. The immunologic status of a population, determined by the ratio of resistant to susceptible members and their distribution.
Outbreak investigation
1. define numerator (cases)
a. case definition
2. define denominator: population at risk
3. Calculate attack rate (incidence rate of people at risk)
Attack rate:
# at risk with disease/total # at risk

(%)
Secondary attack rate:
# contacts who develope disease/
#total susceptible contacts

this also is a measure of tendency of spread in a population
Attack rate ratio:
Attack rate % for sick students who ate fish /Attack rate % for sick students who did not eat fish.

A good rule of thumb for determining a relationship for attack rate ratios (or risk ratios) in foodborne disease:
>5 Very strong association (increased risk)
3.0 -<5.0 Strong association (increased risk)
1.7 -<3.0 Moderate association (increased risk)
1.3-1.6 Weak association (increased risk)
0.9-1.2 Probably no association
0.5-0.8 Weak association (decreased risk)
Incubation Period
Interval from recipt of infection to the time of onset of clinical illness
susceptibile
the probability to get disease (NEVER 0)
non-susceptible
the probabilty to get disease IS ZERO
Steps to epidemiology study
1. Define disease
2. Define population
3. Find all cases in the population (existing and new)
4. Create measures of case frequency per population
Prevalence (A SNAPSHOT)
# the total number of cases of a given disease in a specified population at a specified time and/or
# the ratio of the number of cases of a disease present in a statistical population at a specified time and the number of individuals in the population at that specified time.

NOT a measure of risk and NOT a rate
Point prevalence
How many on a certain date? aka cross-sectional sample
Period prevalence
disease cases present during a specific time interval (NOT a good measure)
prevalence case bias
bias due to longer disease survivability
RATES
proportions, ratios, risk and instataneous rates over TIME
Proportion
includes the numerator in the denominator
ratio
numerator and denominator come from different groups (male/female)
Risk
result of rates that prevail over a period of time. Time is not a dimension; only used descriptively to specify a period of observation
Cumulative incidence CI:
number of new cases divided by the candidate population over a period of time.
also be calculated by the incidence rate multiplied by duration.
CI(t)=1-e^{-IR(t) * D}
new cases/initial population at risk
Incidence Rate IR (density)
new cases/at risk time

"stroke incidence for males is 5 cases/100,000 person-years
Incidence Density ID
new cases/person-years
Crude rates
total number of something per 1000 people. summary for a population of comparison age group. Not used for inter-population comparisons.
standardised mortality rate (SMR) or age-specific mortality rate (ASMR)
total number of deaths per 1000 people of a given age (e.g. 16-65 or 65+).

Factors affecting a country's death rate

* Nutrition levels
* Standards of diet and housing
* Access to clean drinking water
* Hygiene levels
* Levels of infectious diseases
Specific Rates
ALWAYS can be compared between groups.
Direct Standardization of rates
1. choose a standard population
2. multiply specific rates from pop#1 by standard pop age groups
3. sum the pop#1 and divide by total standard pop, then repeat for #2.
ADJUSTS FOR CONFOUNDING EFFECT
Protortionate Mortality Ratio (PMR)
observed deaths in population A/expected deaths based on the proportion in the population B.
Used when actual population numbers are not known. NOTE: NOT rates or risks
Crude Birth rate
#live births in a year/average midyear population x 1000
crude death rate
#deaths in a year/ave mid-year pop x 1000
Age-specific death rate
#deaths in certain age in a year/ave pop in age group x 100
cause-specific death rate
#deaths from cause/mid-year pop x 1000