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What is a Cohort?

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General Term = group of people who share a common experience ----Persons born in same year- birth cohort ----Persons who share a common behavior e.g., cohort of smokers; employees at the tire manufacturing plant ----Persons in the same class- e.g., MPH cohort of spring, 2013

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Cohort Studies

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Exposed and unexposed individuals are followed (forward in time) to determine the incidence of disease in each group

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3 Types of Cohort Study Designs

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1.) Prospective 2.) Retrospective 3.)Ambi-directional

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Prospective Cohert

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(Concurrent/Longitudinal) Investigator collects information on the exposure status of study subjects at the time the study begins and identifies new cases of disease that develop from that time on, until the end of the follow-up interval.

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Retrospective Cohert

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(Non-concurrent / historical) Investigator determines exposure status from information recorded at some time in the past, and disease status is determined from that point in the past up until the present (i.e., the follow-up period has already occurred)

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Ambi-directional Cohort

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(Combined nonconcurrent/concurrent) Cohort and exposure status identified from past records, followed into the present, then followed into the future. Most often used when additional follow-up time is needed.

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Selection of Study Population

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. General population sample - Representative sample from the general population Makes results highly generalizable Expensive, labor intensive, and may have problem with loss to follow-up. E.g., Framingham; Strong Heart Study . Special cohort - Defined population based on membership in a particular subgroup of interest. Follow-up may be easier Results may not be as widely generalizable. E.g., Nurses Health Study; U.S. Veterans Presence of a Distinctive Exposure - Selected because they are known to be exposed to a certain factor Generally p used for occupational cohorts. Cohorts may be stable and easy to follow. Results may not be generalizable. E.g., Atomic bomb survivors; persons exposed to chemicals on the job; licensed anglers

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What is the Reference Group for the Exposed?

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. Internal Comparison - from the same population as the exposed group e.g., unexposed in the population (Framingham, low cholesterol levels) 2. External Comparison - outside of the exposed group e.g., general population data; another study's data 3. Combined - both internal and external comparison groups can be used

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Cohort Characteristics

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Population based vs. Non-population based Open cohort (persons enter and leave over the course of follow-up) vs. Closed cohort (begin with a fixed study group; persons may leave but no new members enter)

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Assessment of Exposure

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1.Definition of exposure - what will constitute "exposure"? 2. Sources of exposure data • It i nervew • Existing records • Physical exam 3. Timing of exposure - onset 4. Quantifying exposure • Frequency - how often? Continuous or intermittent? • Intensity - how much? • Duration - how long? 5. Changes in exposure status - how to deal with?

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Measurement of Disease

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Determine disease-free status at start of study Outcomes must be clearly defined and measurable - who is an incident case? How will you determine whether or not disease has occurred? Procedures should be well-defined and the same for both exposed and un-exposed groups Best if assessment of disease incidence is done blinded to exposure status

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Measurement of Affiliation of Cohort Studies

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Cumulative Incidence!!! RISK RATIO / RELATIVE RISK

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Outcome Measures in Cohort Studies: Cumulative Incidence

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CI: all cases known to have occurred in the baseline cohort during the follow-up time divided by the study population at baseline, per unit of time. Numerator is # of incident cases; Denominator is # people in study population Closed cohort CI is a risk measure ID: all cases known to have occurred in the baseline cohort during the follow-up time, divided by the amount of "at risk" experience (usually in units of person-time) contributed by all members of the cohort Numerator is # of incident cases; Denominator is a blend of # people and their time at risk Open or closed cohort ID is a rate measure

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Risk Ratio (RR) (Relative Risk)

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CI(E-exposed)/ CI (noE-non-exposed) = RR (a/(a+b))/(c/(c+d)) = RR

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Interpretation of RR

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RR = 1.0 No association between exposure and outcome RR > 1.0 Exposed are at a higher risk of the outcome than the not exposed ( i i i i ) d (positive association) RR < 1.0 Exposed are at a lower risk of the outcome than the not exposed (negative association; preventive factor)

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Attributable Risk (AR) in the Exposed

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How much of the disease in people who are exposed is due to the exposure? (AR in the exposed = absolute difference in incidence) What percentage of the disease in the exposed is due to the exposure? (AR% in the exposed =percentage)

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Population Attributable Risk (PAR)

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How much of the disease in the population is due to the exposure? ( )PAR = absolute difference) What percentage of the disease in the population is due to the exposure? (PAR% = percentage)

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Advantages of Cohort Design

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1. More certain of temporal relationship between exposure and disease 2. Multiple effects of a single exposure can be assessed 3. Bias in ascertainment of exposure is minimized (i.e., cannot be biased by knowledge of outcome) 4. Can more easily assess changes in risk factor status 5. Direct measurement of incidence of disease

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Limitations of Cohort Design

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Should initially include individuals free of disease, but disease process may have already begun, but not yet detected 2. Expense and time required may limit feasibility 3. Loss to follow-up 4. Not suitable for rare diseases (requires too large a cohort)

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Summary of Cohort Design

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Disease incidence is compared in exposed and un-exposed Comparisons for exposed group's experience can be internal, external or both Two measures of incidence can be calculated: Cumulative incidence (measures risk of disease) Incidence density (measures rate disease incidence) Primary measures of association from a cohort study are: Relative risk or Rate ratio (RR) = 1 0 i di t i ti b t .0 indicate no association between exposure and disease incidence >1.0 suggest positive association <1.0 suggest protection (negative association) CAN ALSO CALCULATE OR

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Case Control Study (What does it do?)

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Selects the study population on the basis of disease status A case-control study begins with people who have the disease (cases) and compares them to people who don't have the disease (controls) Compare the odds of past exposure to a suspected risk factor between cases and controls

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Case Control Study Example

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Suppose we are interested in investigating an association between childhood cataracts and exposure to rubella virus in utero. Cases would be children with cataracts Controls would be children without cataracts For each child we would determine whether or not their mother was exposed to rubella during her pregnancy with that chi

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Issues in Case Selection: Definition

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Diagnostic criteria - Clearly defined, objective, standardized criteria Objective: to produce a uniform, homogenous group of cases Example: In a case-control study of preterm delivery, cases would be identified as babies born < 37 weeks of gestational age as defined by first trimester ultrasound measurement of crown-rump length. Criteria for eligibility - Clearly defined reasons to include/exclude cases E.g., by age, gender, potential for exposure Apply equally to cases and controls Example: A study of recent OC use and MI would exclude males and postmenopausal and surgically sterilized women because they have no risk of recent exposure. Including them would bias the results towards the null.

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Case Control: Incident vs Prevalent Cases

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Prefer to study incident cases Why incident cases? -Reduce potential for I/P bias (P≈ IxD etiology v. survival/duration) -Diagnoses more likely to be uniform, using same criteria -Recall of exposure may be better

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Case Control Case Selection

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Cases are selected without reference to (without knowledge of) their exposure history (Base it on disease)

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Case Control Sources of Cases (Hospital Based)

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all cases admitted to a single hospital or group of hospitals within a given time period who meet the eligibility criteria Advantages: easier, cheaper Disadvantages: • Potential for biased sample of cases • Referral patterns • Only suitable for diseases that are usually hospitalized

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Case Control Sources of Cases (Community or Population Sample)

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include all (or random sample) cases in defined geographical area within specified time period; use registry if it exists Advantages: representative case group Disadvantages: • Costly • Time-consuming • Difficult to do without registry

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Sources of Cases (Other Sources)

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Choices of case source depends on the disease/condition, especially if it is a one for which people are not uniformly hospitalized Other Sources: -Registries which are not population-based -Large pre-paid insurance plans -Retirement communities

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Selection of Controls

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Purpose: Controls provide a comparison group for cases Definition: Controls intended to rep qy present the frequency of exposure in the population from which the cases arose Controls are selected from the same source population as the cases were chosen Controls are free of disease under study Controls usually similar to cases with regard to past potential for exposure, during same period of risk under study Usually select controls in same manner as cases selected in order to select from the same source population Eligibility criteria - any exclusion criteria applied to the cases also applied to the controls

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Selection of Control Example

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A case-control study of tonsillectomy and lymphoma conducted in Iowa would select cancer cases from the state-wide SEER cancer registry. Because cases are identified on a statewide basis, controls should also be chosen on state-wide basis. The "source population" for cases is the State of Iowa.

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Again Control Selection

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Controls are selected WITHOUT REFERENCE TO THEIR EXPOSURE HISTORY WITHOUT KNOWLEDGE OF EXPOSURE HISTORY

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Community/General Population Controls

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Controls are selected from a random sample of the general population Random-digit dialing commonly used Appropriate if cases are population based

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Advantages/ Disadvantages of Community/General Population Controls

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Advantages: Highly representative; calculate population frequency of exposure appropriate for population based cases Disadvantages: Costly, problems with refusal and phone coverage

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Hospital-Based Controls

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People seeking medical care at same institution as cases for conditions unrelated to disease under study Exclude persons with diseases known or suspected to be related to exposure under study The illness of the controls should have the same referral patterns to the health care facility as that of cases May use multiple diagnoses

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Advantages/Disadvantages of Hospital-Based Controls

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Advantages: Captive population, clearly identified, economical method, less recall bias Disadvantages: Potential for selection bias, less generalizable

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Neighborhood Controls

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-Controls Selected from Same neighborhood as cases through canvassing or use of phone number

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Advantages/Disadvantages of Neighborhood Controls

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-Advantages: Provides controls of similar socioeconomic status and environment as cases -Disadvantages: Overmatching possible, low response rates

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Multiple Types of Controls

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-Used to assess potential biases (e.g. recall) or to compensate for the deficiencies of other types of controls

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Advantages / Disadvantages of Multiple Types of Control

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-Advantages: Internal replicate of study: assess certain types of bias -Disadvantages: Costly and time-consuming, if results differ, must be able to explain

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Example of Case Control

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A case-control study of brain tumors in children included two different types of controls: Children with other solid tumors ("sick controls") A random sample of healthy children from the same birth cohorts as cases (community controls) Purpose: to assess results for potential recall bias

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Case Control Question

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In a case-control study of breast cancer and pesticide exposure in Oklahoma, cases of breast cancer are chosen from the tumor registries of all major hospitals in Oklahoma County. What is the "source population" for these cases? How should controls be picked?

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Representativeness Vs. Comparability

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Is goal to select cases and controls that are representative of those with and without the disease, or to select cases and controls so they are like each other in all ways relevant?

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Representativeness

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Relates to the generalizability of the study Advantages: increase generalizability, reduces potential for certain types of selection bias Disadvantages: may be more difficult, time consuming, resource intensive

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Comparability

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Internal validity; cases and controls have equal probability of past exposure if there is no association between exposure and disease. Advantages: easier to detect smaller differences; reduces chances for unmeasured confounding Disadvantages: may have select group of cases, potential for Berkson's bias (hospital-based studies only)

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Assessment of Exposure

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Exposure is prior to disease onset (or reference date for controls) Techniques: Information on prior exposure in cases and controls may be ascertained through personal interview, hospital or medical records, employment records, pharmacy or lab records, or direct measurement Timing of exposure - when did it occur in relation to disease onset or index date? Quantification of exposure for dose response analysis Includes amount, duration and frequency of exposure.

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Assessment of Exposure Example

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Weinmann et al. (1994) conducted a case control study of renal cell cancer in relation to antecedent use of antihypertensive medications within the memb hi f K i ership of Kaiser-P t ermanente Northwest. For cases and their matched controls, outpatient and inpatient medical records were reviewed for information regarding medication use up to a date three months prior to the case's diagnosis

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Measure of Association in Case Control Studies

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ODDS RATIO-CAN'T DO RR, NO CI (incidence) Odds Ratio (OR) Ratio of two odds (the odds that cases were exposed ÷ the odds that controls were exposed) What is an "odds"? The odds of an event is defined as the probability of an event occurring (P) divided by the probability of the event not occurring (1-P) Odds = P ÷ (1-P) In other words, odds that cases were exposed = probability that cases were exposed ÷ probability that cases were not exposed. This is one of the two odds that make up the odds ratio.

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Example of Odds

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If there are 100 smokers and 60 develop a chronic cough, the probability of smokers developing a cough is 60 / 100 = 60% The probability of smokers not developing a cough is 100% - 60% (or 40 / 100) = 40% The odds of developing a cough are 60:40 or 1.5 Note how the odds of developing a cough differs from the probability of developing a cough

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Odds Ratio (How to do it?)

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ac/bd

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Interpretation of OR

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The odds of having smoked cigarettes among cases of lung cancer are 14.0 times greater than the odds of having smoked cigarettes among controls. OR = 1.0 No association between exposure and outcome OR > 1.0 Positive association between exposure and outcome OR < 1.0 Negative or inverse association between exposure and outcome

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Relative Risk?

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You cannot, in most circumstances, use case-control data to calculate incidence and thus, you cannot directly calculate relative risks from case-cont l d t rol data Study groups are chosen on the basis of presence of disease, not exposure Odds ratios are good approximations of the relative risk if your study is properly designed

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Matching of Cases and Controls

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Definition: The process of selecting controls so that they are similar to cases for characteristics, such as age, gender, race, or socioeconomic status, that might be confounding variables. Matching is one of the ways to deal with confounding, by making cases and controls similar across these characteristics.

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Types of Mismatching (Individual)

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Individual (pair) matching - selecting one or more controls for each individual case e.g., for a case of MI who was a white male aged 67 years, select one or more controls without MI who are white, male and 62-72 years of age - repeat for each case in the study

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Types of Mismatching (Frequency (group) Matching)

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Frequency (group) matching - the control group is selected so that its distribution is similar to that of cases for py gotentially confounding variables such as age, gender and race. Requires that all of the cases be selected first or that the distribution is known. e.g., if 68% of your MI cases are male, 68% of your controls are chosen to be male

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Interpretation

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The odds of having been born high birth weight are 2.57 times higher in children with brain tumors than in children without brain tumors.

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Advantages of Case Control

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. May be quicker and less expensive compared to cohort studies. 2. Well suited for rare disease and diseases with long latent periods. 3. Fewer subjects required than for cohort studies. 4. Multiple etiologic factors can be evaluated.

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Disadvantages of Case Control

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1. More potential sources of bias and error than cohort studies 2. Temporal relationship - did exposure cause disease or did disease cause exposure? 3. Appropriate control group may be difficult to identify.

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Summary of Case Control

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Frequency of past exposure is compared in persons with the disease (cases) and persons without (controls) Cases and controls should be selected without any knowledge of their exposure history Controls provide an estimate of the exposure frequency in the population from which the cases arose Measure of association from a casecontrol study = odds ratio Ratio of the odds of exposure in cases to the odds of exposure in the controls =1.0 suggests no association between exposure and disease >1.0 suggests a positive association <1.0 suggests a protective (negative) association

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