##### Intro to Epidemiology - Lectures 2
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#### Complete List of Terms and Definitions for Intro to Epidemiology - Lectures 2

Terms Definitions
Is there follow-up in a cross-sectional study? No.
Epidemiology is... Epidemiology is the discipline concerned with the study of the frequency of occurrence of illness (disease, defect or injury)
John Snow London physician in 1800s, studied cholera outbreak and reasoned it came from contaminated water. Studied death from cholera based on company of water supply (two water supply companies in London, randomly distributed. Natural study.)
What are the different types of determinants? -Constitutional
-Behavioral
-Environmental
What are the two overarching categories of epidemiologic study designs? Experimental and observational
What are other names for Cumulative Incidence? -Incidence proportion
-Risk
What is association? The relation of an occurrence parameter to one or more characteristics of persons

Association DOES NOT equal causation
What is a p-value? The probability of obtaining results as or more extreme, by chance alone, given that H-null is true.
What is estimated in a case-control study? Incidence odds ratio
Odds index cases / index controls = odds of illness among index (exposed)

reference cases / reference controls = odds of illness among referent (unexposed)
What conditions are necessary to estimate prevalence from incidence? Steady State conditions:
-Incidence rate is constant over time (stable)
-Duration of illness is stable over time
-Inflow of illness = outflow of illness
What is descriptive Epidemiologic research? Research that provides descriptive information on patterns of occurrence of illness, such as age, sex, geographic location, time, etc.
What is an index group? Individuals with a particular characteristic of interest

(Often referred to as the exposed group)
what are the advantages of intent-to-treat analysis? Maintains randomization, groups are balanced
Why conduct a case-control study? -Efficient design to study an etiologic question, especially when:
1.Illness is rare
2. Exposure is difficult or expensive to obtain
3. Disease has a long time between exposure and disease
4. Difficult to follow and track individuals in a particular dynamic population
AF for case control study?

PAF?
AF = (OR-1)/OR

PAF = Pe*(OR-1)/OR
Pe = # of exposed cases/total number of cases
What are two different types of RCTs? Treatment allocated to INDIVIDUALS

Treatment allocated to ENTIRE COMMUNITIES (number of communities, not individuals, constitutes the study size
Relative measures of association interpretation =1: No association between determinant and illness
&gt;1:determinant associated with increased risk/incidence rate/prevalence
&lt;1: determinant associated with decreased risk/incidence rate/prevalence
What is the determinant (exposure)? A characteristic of individuals on which the frequency of occurrence of illness depends either causally or non-causally.
What is Cumulative Incidence? The proportion if individuals in a population who develop the event of interest in a specified time period.

(The population is free of the event at the start of follow-up!_
What is scientific Epidemiologic research? Research that aims to advance theoretical knowledge about nature and pertains to causality.
What does a cross-sectional study allow for the estimation of? The prevalence ratio and prevalence difference.
Where do the controls come from ion a secondary study base? Examples:
-Family, friends
-Residents of the same neighborhood
-Patients admitted to the hospital with other illness unrelated to the determinant of interest
-Patients attending the clinic for other illness unrelated to determinant of interest
What is an observational study design? One in which the determinant/exposure is determined by some other mechanism (non-assigned).

-cohort
-case-control
-cross-sectional
-Ecologic
What are the strengths of a cross-sectional study? (there are 2) -Relatively quick and inexpensive

- Good for early stages of knowledge
What types of studies can be conducted in a cohort? Cohort study designs
cross-sectional study designs
case-control study designs
What is the source population? The population from which the study population and base is selected by use of exclusions.

(the pool from which we're pulling people)
When is cumulative incidence usually estimated? In fixed populations with relatively complete follow-up

(Assumes everyone in population is followed for specified time period)
When is Incidence Rate usually estimated? -In dynamic populations
-In fixed populations with considerable losses to follow-up (because it takes follow-up time changes into account)
How is the outcome assessed in a cohort study? Many different ways (examples below):

-Medical or occupational records
-Routine examinations at cohort visits (lab tests, physical measurements)
-Questionnaires
In an RCT, what forms the study population and study base? Enrollment into the cohort.

Involves follow-up of cohort members.
What are the potential strengths of using a primary study base? -It's clear that controls represent the study base (population experience) that produced the cases (same source population)

-In a case-control study conducted in a cohort: data and specimens often already drawn and stored (quick and inexpensive to conduct as long as specimen quality not an issue)
What are the potential limitations of using secondary study bases? -Difficult to know if controls represent the study base that produce the cases (come from the same source population)

-May be difficult to select control diseases unrelated to determinant of interest
What is a primary study base? -The source population that gave rise to the cases is KNOWN FIRST and THEN cases are identified

(Cases come from a population that is explicitly and directly defined)

The study base is defined directly (and not indirectly through case identification)
What is an environmental determinant? A determinant having to do with the external conditions or surroundings of an individual

E.g. Air quality, in utero exposure to a particular medication
What is the Cumulative Incidence formula? # of individuals who develop the illness event during the time period / # of individuals followed during the time period
What is the interpretation of absolute measures of association? =0: no association between determinant and illness
&gt;0: determinant associated with increased risk/incidence/prevalence of illness
&lt;0: determinant associated with decreased risk/incidence/prevalence of illness
What is a causal association? The frequency of illness is related to the determinant and may be viewed in causal terms
What is a descriptive association? The frequency of illness is related to the determinant without any view to causal interpretation
What is the population attributable fraction? The proportion of cases in the POPULATION caused by the exposure
PAF = Pe*AF
where Pe=# of exposed cases/total number of cases
Why would an external reference of another cohort group be used? -Often used in studies of occupational exposures --&gt; Another occupational cohort could share some behavioral risk factors

-A working reference cohort eliminates the Healthy Worker Effect
What are some limitations to using a primary study base in a case-control study? -Expensive to obtain controls: random digit dialing
-Controls tend to be healthy so:
a. They can be less interested in participating and lower response rate (selection bias)
b. Accuracy of determinant information may be different than the cases (misclassification bias)
Where do the controls come from in a primary study base? Examples:
-Cohort (in a case-control study conducted within a cohort/fixed population)
-National identity registries
-Town lists
-Random digit-dialed telephone numbers
What are the limitations of cohort studies? -Costly for rare outcomes or illnesses with long induction periods
-Expensive to follow individuals, collect data, conduct routine medical examinations and lab tests etc
-can have loss to follow-up
How is a secondary study base defined? -Cases come from a population that cannot be explicitly defined
(We identify cases first then try to identify the source population that gave rise to the cases)

-The study base is INDIRECTLY defined; it is defined secondarily to the identification of the cases
What is differential loss to follow-up and what does it affect? Differential = related to both outcome and determinant

Affects study validity (selection bias)
When one initiates a cohort, what does it involve? Follow-up of people, data collection, etc. for the occurrence of illness (or change)
What are the disadvantages of efficacy analysis? Lose randomization, so groups may not be balanced
What is the interpretation of a confidence interval? &quot;The interval was produced by a method that contains the true value 95% of the time in the absence of bias.&quot;
What is the study population in an RCT? Formed by enrollment into the cohort
What are the two main criteria for valid control selection in a case-control study? -Controls come from the source population that produced the cases; if they develop illness, would be a case in the study

- Controls are selected INDEPENDENT OF EXPOSURE
What are the defining features of a cohort study? A cohort study involves follow-up (documentation) of cohort members over time.

-It's a study of incidence (e.g. diagnoses) or change (e.g. progression or improvement of illness)
What does a normal 2x2 table look like? -Determinant (Index and Reference in that order) in columns
-Rows = cases then denominator
What are the dimensions of Epidemiologic Research? Population - a set of members
Time - temporal aspect
What units are associated with prevalence? None!
Answer will be between 0-1 or 0-100%
What is included in Follow-Up time? Only the time at risk of developing the event
How can we maintain follow-up (study retention)? -Make it easy for the subject to participate and update you
-Incentives
-Maintain contact with the subject (study visits, questionnaires, study updates, cards, keep notes on contact, update addresses, etc)
How is the determinant assessed in a cohort study? Lots of different ways, just like outcome:

-Medical or occupational records
-Routine examinations at cohort visits (lab tests, physical measurements)
-Questionnaires

Determinant may be fixed (blood type) or may change over time (smoking)
what is a run-in period for an RCT? Certain number of weeks on treatment to find compliant subjects. Only use compliant subjects (randomize after suitable amount of time)
What are the strengths of using an internal reference group? It's the best reference group when feasible

Comes closest to the &quot;counterfactual ideal&quot; because it is most comparable to the index (exposed) group
What is the Incidence Rate formula? IR = # of individuals who develop the illness event / Total time experienced by the population followed
What is one caution about using an external reference group of another cohort? -Need to be careful that the reference cohort doesn't have its own characteristics that may affect illness
When is the outcome assessed in a cross-sectional study? At a SINGLE point in time. (A study of PREVALENT cases)

Determinant usually assessed at a single point in time.