•Definition: A study in which two or more groups of people that are free of disease and that differ according to the extent of exposure (e.g. exposed and unexposed) are compared with respect to disease incidence
Fill in blanks.
Cohort studies are the observational equivalent of__________ but the researcher cannot _________- he must locate ________ to observe the relationship between __________
Cohort studies are the observational equivalent of experimental studies but the researcher cannot allocate exposure - he must locate a natural experiment to observe the relationship between the exposure and disease
What are the Principles of experimental studiesapplied to observational cohort studies
1. Randomization of treatment so groups are comparable on known and unknown confounders. Can't randomize in an observational study so select a comparison group as alike as possible to the exposed group
2. Use placebo in order to reduce bias. Can’t use placebo in observational studies so you must make the groupsas comparable as possible.
3. Blinding to avoid bias in outcome ascertainment.
In a cohort study, it is crucial to have high follow-up rates and comparable ascertainment of outcomes in the exposed and comparison groups.
You can blind the investigators conducting follow up and confirming the outcomes.
What are the types of timing of cohort studies?
•Retrospective: both exposure and disease have occurred at start of study
Exposure---------Disease *Study starts after disease
•Prospective: exposure has (probably) occurred, disease has not occurred Exposure----------------------Disease *Study starts at exposure
•Ambi-directional: elements of both
How do you choose between a retrospective and a prospective design?
•Efficient with diseases with long latent period
•Exposure data may be inadequate
•More expensive, time consuming
•Not efficient for diseases with long latent periods
•Better exposure and confounder data
•Less vulnerable to bias
List examples of exposed populations
Groups undergoing particular medical treatment
Groups with unusual dietary or life style factors
Professional groups (nurses, doctors)
Students or alumni of colleges
Geographically defined areas (e.g. Framingham)
What is the selection of the exposed population in a cohort study dependent upon?
hypothesis under study and feasibility considerations
True or False
For rare exposures, you need to assemble special cohorts (occupational groups, groups with unusual diets etc.)
True or False
If exposure is rare, you may want to use a general cohort that will facilitate accurate and complete ascertainment of data (Doctors, nurses, well-defined communities)
If exposure is common, you may want to use a general cohort that will facilitate accurate and complete ascertainment of data (Doctors, nurses, well-defined communities)
What is the Selection of comparison (unexposed) group Principle:
Selection of comparison (unexposed) groupPrinciple: You want the comparison (unexposed) group to be as similar as possible to the exposed group with respect to all other factors except the exposure. If the exposure has no effect on disease occurrence, then the rate of disease in the exposed and comparison groups will be the same.
Counterfactual ideal: The ideal comparison group consists of exactly the same individuals in the exposed group had they not been exposed. Since it is impossible for the same person to be exposed and unexposed simultaneously, epidemiologists must select different sets of people who are as similar as possible.
List the Three possible sources of comparison group
1. Internal comparison: unexposed members of same cohort
Ex. Framinghame study, Ranch Hand Study
2. Comparison cohort: a cohort who is not exposed from another similar population
Ex. Asbestos textile vs. cotton textile workers
3. General population data: Use pre-existing data from the general population as the basis for comparison. General population is commonly used in occupational studies. Usually find healthy worker effect
Ex. A study of asbestos and lung cancer with U.S. male population as the comparison group
Which of the three comparison groups is best?
List Sources of Exposure Information
Pre-existing records - inexpensive, data recorded before disease occurrence but level of detail may be inadequate. Also, records may be missing, usually don't contain information on confounders
Questionnaires, interviews: good for information not routinely recorded but have potential for recall bias
Direct physical exams, tests, environmental monitoring may be needed to ascertain certain exposures.
List sources of outcome information:
•Physician, hospital, health plan records
•Questionnaires (verify by records)
What is the goal of cohort studies? And how can it be optained?
Goal is to obtain complete follow-up information on all subjects regardless of exposure status. You can use blinding (like an experimental study) to ensure that there is comparable ascertainment of the outcome in both groups.
What are approaches to follow-up?
In any cohort study, the ascertainment of outcome data involves tracingor following all subjects from exposure into the future
Resources utilized to conduct follow-up: town lists, Polk directories, telephone books; birth, death, marriage records; driver's license lists, physician and hospital records; relatives, friends.
This is a time consuming process but high losses to follow-up raise doubts about validity of study
How do you analyze cohort studies?
•Basic analysis involves calculation of incidence of disease among exposed and unexposed groups.
•Depending on available data, you can calculate cumulative incidence or incidence rates.
•Recall set up of 2 x 2 tables.
How is the 2x2 table setup?
exposed A B
non-exposed C D
•Rate ratio = a / (a + b)
c / (c + d)
attack rate of exposed
attack rate of non-exposed
Compute the 2x2 table for this
Example: Tuberculosis treatment and breast cancer study
Followed 1,047 women who were treated with air collapse therapy and exposed to numerous fluoroscopic examinations (radiation) and 717 who received other treatments. A total of 47,036 woman-years of follow-up were accumulated during which 56 breast cancer cases occurred.
What is rate difference and how is it computed?
Rate of illness in the exposed minus the rate of illness in the unexposed
Ire – Irue = Rate Difference (RD)
Ire = Incidence rate exposed
Irue = Incident rate in Unexposed
RD also called “Attributable Risk”
What are the strengths of cohort studies?
•If retrospective, efficient for rare exposures, diseases with long induction and latent period
•Can evaluate multiple effects of an exposure
•If prospective, good information on exposures, less vulnerable to bias, and clear temporal relationship between exposure and disease
What are the weaknesses of Cohort Studies?
•Inefficient for rare outcomes
•If retrospective, poor information on exposure and other key variables, more vulnerable to bias
•If prospective, expensive and time consuming, inefficient for diseases with long induction and latent period
•Keep these strengths and weaknesses in mind for comparison with case-control studies
What type of bias is often found in cohort studies?
–“healthy Worker Effect”
–Loss to follow up
What is selection bias?
•Healthy Worker Effect occurs in cohort studies when workers represent the exposed group and a sample from the general population represents the unexposed group. This is because on average workers tend to be healthier than the general population
What is attrition?
Loss to follow up is the biggest concern in a cohort study. This is where the researcher losses contact with study participants resulting in the loss of data from these individuals. There can be many reasons for individuals to drop out of a study. As a prospective cohort goes on, the risk of attrition increases.
What are confounding factors?
Confounding can influence associations in any study design. It’s where another factor is associated with both the exposure and the outcome under study. If not controlled for this third confounding factor will skew the study results resulting in erroneous conclusions.
What is misclassification?
Misclassification of either the exposure or the disease can occur. If misclassification of the outcome is related to the exposure then misclassification is non-random, differential, and the strength of the rate ratio or risk ratio is distorted. If misclassification of the outcome is not related to the exposure then misclassification is non-differential, random.