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What are two ways to measure disease frequency?

Counts  # with disease and rates counts with a denominator (size of population at similar risk). Rates also imply or are associated with a period of time.


What is the numerator and denominator for incidence

Num = number of NEW cases in a period of time
Den = number of people INITIALLY at risk 

What are the other names for incidence?

Cumulative incidence
incidence rate attack rate risk of disease Probability of getting disease 

How is incidence density different?

The numerator is the same: number of new cases in a time. However, the denominator is "total amount of persontime at risk contributed during the time."
Incidence density accounts for the time someone contributes to the denominator (population at risk for disease) before they become part of the numerator (counts of diseased) 

Define the numerator and denominator of prevalence

Numerator = number of EXISTING cases of the disease at a specific POINT IN TIME
denominator = size of the population at the SAME POINT IN TIME If data is for point in time it is point prevalence. If data is for a specified time. It is period prevalence Period prevalence is different from incidence because the numerator in incidence includes only new cases where is the numerator in any prevalence is all cases. 

Define disease specific Mortality Rate

Numerator = number of people DYING in a specific time.
Denominator = average number people ALIVE during that period of time 

Define Case Fatality Rate

Numerator = number people who die of the disease
Denominator = total number people who get the disease Case Fatality Rate measures disease prognosis rather than disease frequency Some board questions will ask about disease frequency when you are given disease prognosis in the question stem. If this is the case you cannot answer the question with the information given. 

Define Proportional Mortality

Numerator = number of deaths due to a disease in a specific time.
Denominator = total number of deaths during that time. In Proportional Mortality the denominator is the total number of DEATHS. In Disease Specific Mortality the denominator is average number of people ALIVE. 

Define Mortality Rate

Numerator = number of people DYING in a specific time.
Denominator = average number people ALIVE during that period of time 

Define Case Fatality Rate

Numerator = number people who die of the disease
Denominator = total number people who get the disease Case Fatality Rate measures disease prognosis rather than disease frequency Some board questions will ask about disease frequency when you are given disease prognosis in the question stem. If this is the case you cannot answer the question with the information given. 

Define Proportional Mortality

Numerator = number of deaths due to a disease in a specific time.
Denominator = total number of deaths during that time. In Proportional Mortality the denominator is the total number of DEATHS. In Disease Specific Mortality the denominator is average number of people ALIVE. 

Define Mortality (also known as disease specific mortality rate)

Numerator = number of people DYING in a specific time.
Denominator = average number people ALIVE during that period of time 

Define Proportional Mortality

Numerator = number of deaths due to a disease in a specific time.
Denominator = total number of deaths during that time. In Proportional Mortality the denominator is the total number of DEATHS. In Mortality the denominator is the avg number of people ALIVE. 

Define Case Fatality Rate

Numerator = number people who die of the disease
Denominator = total number people who get the disease Case Fatality Rate measures disease prognosis rather than disease frequency Some board questions will ask about disease frequency when you are given disease prognosis in the question stem. If this is the case you cannot answer the question with the information given. 

KEY RELATIONSHIP

Prevalence = Incidence x Duration
This equation is true when prevalence is less than 10% and incidence and duration are stable over time. This is important for predicting. Prevalence will increase if incidence or duration increase. 

KEY RELATIONSHIP

Mortality = Incidents x Case Fatality
This is true when incidence and case fatality are stable over time. 

Define Neonatal Death Rate

Numerator = annual number of deaths in the first 28 days of life
Denominator = annual number of live births 

Define Infant Death Rate

Numerator = annual number of deaths in the first year of life
Denominator = annual number of live births 

Define years of potential life lost (YPLL)

It is a measure of premature mortality.
# deaths X (75  median of age range) 75 is the avg age at death Median of age range is middle age of the group you are concerned with. ex: 010 year olds = 5 So if 2 kids in the age group 010 die then 2 (755) = 140 YPLL 

Define years of potential life lost (YPLL)

It is a measure of premature mortality.
# deaths X (75  median of age range) 75 is the avg age at death Median of age range is middle age of the group you are concerned with. ex: 010 year olds = 5 So if 2 kids in the age group 010 die then 2 (755) = 140 YPLL 

Be careful of lifetime risk estimates that don't give you that life expectancy on which they were based

For example: one in 11 women get breast cancer if they live to 75 (the average life expectancy females)
often quoted is that one in eight women get breast cancer but this is based on a life expectancy of 90 years. 

What are critical factors when interpreting the disease frequency measures of epidemiology (counts and rates)

You must consider the source of your numerator and denominator.
Some numerator data (counts) are limited by quality of reporting and case definition. Denominator data must condier the kinds of people at risk. Are you including people who are not at risk or only those at high risk? Factors include gender, location, season, disease cycles, etc. 

What is the cohort effect?

Variation in disease rate between different age cohorts. Example. in 1950 50% of men smoked. Therefore mortality from smoking in men alive in 1950 is higher than men not alive in 1950. There will be a migrating spike over time as this cohort ages.


After measuring Disease Frequency (counts and rates) that next common set of epidemiologic measures is the Measure of Excess Risk

These rely on 2 x 2 table computations.
They are measures such as relative risk and odds ratio 

Relative risk (risk, risk ratio, rate ratio) is defined as:

RR =
Numerator = incidence in the exposed divided by Denominator = incidence in the unexposed This is [a/(a+b)] / [c/(c+d)] With disease present on the top of the 2x2 table these calculation go horizontal. 

After measuring Disease Frequency (counts and risks) that next common set of epidemiologic measures is the Measure of Excess Risk

These rely on 2 x 2 table computations.
They are measures such as relative risk and odds ratio 

Relative risk (risk, risk ratio, rate ratio) is defined as:

RR =
Numerator = incidence in the exposed divided by Denominator = incidence in the unexposed This is [a/(a+b)] / [c/(c+d)] With disease present on the top of the 2x2 table these calculation go horizontal. 

When can the relative risk be estimated by the odds ratio?

When the disease is rare.
This means that A and C. are very small values compared to B and C. 

What is the formula for odds ratio:

OR = (a/b) / (c/d) = ad / bc


What is attributable risk

Incidence in the exposed minus incidence in the unexposed.
This is [a/(a+b)]  [c/c+d)] when you divide this quantity by the first term, [a/(a+b)], and multiply by 100 you get Attribute Risk Percent 

Attributable Risk and Attributable Risk Percent interpretation:

is the absolute number of excess cases because of exposure and percent of excess cases excessive exposure


Population Attributable Risk is:

Total incidence minus incidence in the unexposed.
This is the proportion of all case of disease that are due to exposure. 

STUDY DESIGN (concepts of studies)

Bias
Study Types Epidemiologic evidence and causal inference Confounding effect modification Disease screening Infectious disease epidemiology 

Define bias

Bias result of a systematic error in the design or
conduct of a study resulting in false or misleading conclusions. Biases can EITHER falsely raise OR lower the estimates of risks. 

Define random versus systematic error

Random error is noise in your data is nondirectional examples are precision of diagnosis precision of exposure measurement precision of confounder assessment. It does not invalidate a positive result. It may invalidate a negative result.
Systematic error is directional. They are biases. It is an inherent flaw in the study design. It may invalidate either a positive or negative result. 

Name three major classifications of bias

Selection bias  different probabilities of being included in the study
Information bias  systematic tendency for individuals to be MISCLASSIFIED into the wrong exposure or outcome categories Confounding (a specific type of bias) 

What is recall bias

Those without outcome won't remember their exposure. Those with the outcome may exaggerate the amount of their exposure.


What is measurement bias

An example would be misclassification into an exposed group based on the DEFINITION or interpretation of exposure.


What is internal validity

The method of the study truly supports the claim of a causal relationship between exposure and outcome.


What external validity

The strength of the study's generalizability to a population. It can also be thought of as the strength of its inductive reasoning.


What is reliability

Reliability is the repeatability; the extent to which the results obtained by a test is replicated if the test is repeated (accuracy)


What is confounding

Confounding is a third variable related to exposure and disease. It biases the estimate of the exposures affect on the outcome. Common confounders or age, gender, tobacco use, BMI, socioeconomic status.


Studies have a hierarchy. What are the three main levels to this hierarchy.

Descriptive studies followed by observational studies followed by interventional or experiment studies.
Descriptive studies are also described as hypothesis generating Observational and Interventional Studies Are Considered Hypothesis Testing. 

Name two types of descriptive hypothesis generating studies

Case report
Case series Descriptive Epidemiologic Study (describes how something varies with the change in a factors such as time, gender, or location) 

Name four types of observational studies.

Correlational or ecological study
CrossSectional or prevalence study CaseControl study Cohort Study 

What are the two types of cohort studies

Prospective and retrospective


What are the two types of interventional studies

Randomized controlled trials and croissants experimental


Descriptive studies

Lack of offices
do not establish causal associations 

Analytic studies

Usually have hypothesis specified in advance
intent on establishing a causal association The premise is that a variation in exposure is associated with a variation in outcome. 