1. Dr K N Prasad MD., DNB Community Medicine
Cohort study
Dr K N Prasad MD., DNB
Community Medicine

2. Aim of Epidemiological studies
To determine distribution of disease
To examine determinants of a disease
To judge whether a given exposure causes or prevents disease

3. Epidemiological study designs
Descriptive studies
Populations
Correlated studies
Individuals
E.g. case-series, case reports, cross-sectional surveys
Analytical studies
Observational studies
Case-control studies
Cohort studies
Experimental studies
Intervention studies after randomise exposure
Clinical trials
but before I go into details about the cohort and case-control studies I would like to place these kind of studies in relation to other types of epidemiological studies.(klik)
We have descriptive and analytical studies and the analytical studies can be divided into observational and intervention studies.
Cohort and case-control studies falls into the category of analytical, observational studies.
In observational studies, the investigator simply observes the natural course of events noting who is exposed and nonexposed and who has and who has not developed the outcome of interest (Hennekens and Burring pp.22).
(klik)
…and as you see, there are two basic types of observational analytical investigation:
the case-control
and the cohort studies.
In practice each design offers certain unique advantages and disadvantages, and very briefly you can say that case-control studies are well-suited for investigation of rare diseases, whereas cohort studies are well-suited for relatively common diseases.
but in general these study designs require an appropriate comparison group (control group).

4. Case-control study Exposed Cases Non-exposed Study Population Exposed

5. Cohort study / Follow-up study
Study population
Exposed
Non-exposed
Disease +
Disease -
A cohort study is a study where a group of individuals are followed. The study population is defined on basis on the presence or absence of exposure to a suspected risk factor for a disease.
The cohort studies is either classified as a prospective or a retrospective study.
The definition is based on the time of occurrence of the disease according to the initiation of the study.
In the prospective studies the disease of interest is never occurred at initiation, the exposure are either presence at the beginning of the study or it can occur later. The prospective cohort you have to follow over time.
In the retrospective cohort study, all the relevant events disease and exposure have occurred before the initiation of the study.
outcome
follow- up
retrospective/prospective
Protocol: study objectives, design choices, performance goals, monitoring and analysis procedures
Manual of procedures/log book
Centralised training of key personnel

6. General considerations
A cohort :
A group of persons, identified at one point in time, who march off together into the future under the watchful eye of an investigator.
A cohort study:
A group of persons is defined, certain characteristics about each individual are recorded, and they are then followed up in such a way that new events (such as disease and death) or other changes in their characteristics are detected.

7. Cohort Study Longitudinal study, Follow-up study, prospective study
Definition: An analytical epidemiological study in which two or more groups of people according to the extent of exposure (e.g. exposed and unexposed) are compared with respect to outcome or disease incidence
Most reliable for showing an association between a suspected risk factor and subsequent disease

8. Features of cohort study
Cohorts must be free from the disease under study
Both the groups should be equally susceptible to disease under study
Diagnostic and eligibility criteria of the disease must be defined beforehand

9. Cohort study
Exposed and non exposed individuals are followed over time to determine whether they experience the outcome of interest.
Examples of exposure :
Medication use, Environmental factors, condition, Procedure
Examples of outcome:
Disease. Death, etc.

10. Cohort studies Retrospective Prospective Ambidirectional
Exposure Disease
Yes ?
No ?
Prospective
Exposure Disease
Yes ?
No ?
Ambidirectional
A cohort study follows-up two or more groups from exposure to outcome. In its simplest form, a cohort study compares the experience of a group exposed to some factor with another group not exposed to the factor.
For all cohort studies, the subjects are selected according to whether they are exposed or unexposed to the factor under investigation, and their subsequent disease status is ascertained after some time. The follow-up period are usually several years. – Example: The Nurses Health Study
Generally, at the time exposure status is defiend, all potential subjects must be free from the disease under investigation, and eligible participants are then followed over a period of time to assess the occurrence of that outcome.
Researchers doing this kind of study must, therefore, (klik) go forward in time from the present (prospective) or (klik) go back in time to choose their cohorts (retrospective).
…………..
The feature that distinguishes a prospective from a retrospective cohort is simply and solely whether the outcome of interest has occurred at the time the investigator initiates the study.
In a retrospective cohort study, the investigation is initiated at a point i time after both the exposure and disease have already occured.
At the beginning of a prospective cohort study, the groups of exposed and unexposed subjects have been assembled, but the disease has not yet occured, so that the investigator must conduct follow-up during an appropriate interval to ascertain the outcome of interest.
Can you give me an – hypothetical – example of a prospective study?
- Example: The Frammingham Heart Study, Nurses Health Study
A third design obtion is the ambidirectional study: This mean that data are collected both retrospectively and prospectively on the same cohort.

11. Timing of cohort studies
Retrospective: both exposure and disease have occurred at start of study
Exposure Disease
*Study starts

12. Timing of cohort studies
Prospective: exposure has (probably) occurred, disease has not occurred
Exposure Disease
*Study starts
Ambi-directional: elements of both

13. Elements of cohort study
Selection of study subjects( cohorts)
Selection of comparison group
Obtaining data on exposure
Follow up
Analysis

14. Selection of the Exposed Population
Sample of the general population:
Geographically area, special age groups, birth cohorts (Framingham Study)
A group that is easy to identify:
Nurses health study
Special population (often occupational epidemiology):
Rare and special exposure
Permits the evaluation of rare outcomes

15. Selection of the Comparison Population
Internal Control Group
Exposed and non-exposed in the same Study population (Framingham study, Nurses health study)
Minimise the differences between exposed and non-exposed
External Control Group
Chosen in another group, another cohort (Occupational epidemiology: Asbestosis vs. cotton workers)
The General Population

16. Selection of comparison group
Internal comparison group: according to the degrees or levels of exposure
Smokers, BP, Alcohol, diet etc.
External comparison
Similar in all respects without any exposure
Comparison with general population rates
Outcomes are compared with the similar outcome rates in the general population

17. Sources of exposure information:
Pre-existing records - inexpensive, data recorded before disease occurrence but level of detail may be inadequate.
Records may be missing, / usually don't contain information on confounders

18. Sources of exposure information:
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.

19. Follow up Regular follow up of all participants
Periodic medical examination of each member
Reviewing physician and hospital records
Routine surveillance of death records
Mailed questionnaires, telephone call, periodic home visits

20. Sources of outcome information:
Death certificates
Physician, hospital, health plan records
Questionnaires (verify by records)
Medical examinations

21. Analysis in cohort study
Incidence of disease among exposed and non exposed
Relative risk estimation
Attributable risk estimation

22. Table for analysis cohort study
Disease present
Disease absent
Total
Exposure
Present ( cohort) a b
a + b
Exposure absent (comparison) c d
c + d
a + c
b + d
a+b+c+d
Incidence of disease among exposed = a / a+b
Incidence of disease among non exposed = c / c+d
P value should be <0.05

23. Relative risk Relative risk is calculated as
Incidence of disease among exposed
Incidence of disease among non exposed

24. Relative risk (Risk ratio)
Quantifies magnitude of the association between exposure and disease
Varies from 0 to infinity
RR<1: exposure decreases the risk for disease
RR=1: no association
RR>1: exposure is a risk factor for disease; increases risk for disease
Example:
RR=2.0 can be interpreted as two fold increase in risk

25. Attributable risks Also known as risk difference
It is the difference in incidence rates of disease between exposed group and non exposed group.
It suggests the amount of disease that might be eliminated if the risk factor could eliminated or controlled.
Incidence of disease among exposed - incidence of disease among non exposed
x 100
Incidence of disease among non exposed
ex. AR is 90%. Interpretation-

26. Bias in Cohort study
Selection bias - less of a problem than case control studies
Information bias/misclassification
Degree of accuracy of classification of exposure, confounders and disease status
Loss to follow-up (affects validity)
Non response (limits generalisability, not validity)
Confounding

27. Cohort study Limitations Strengths  Loss to follow-up
 Misclassification of disease
or exposure status
 logistically challenging – especially for prospective design
 Hard to study rare diseases
 Changes over time in staff/methods
 Little control over nature
and quality of data in
retrospective designs
Strengths
 Can establish time order
 Can obtain incidence rates
 Can study more than one
disease or outcome
 Minimizes bias in
ascertainment of exposure
status and covariates –
especially if collecting data
prospectively
 Efficient for rare exposures
 No controls, so no bias in
control selection

28. Case-control study Cohort study Rare exposure Quick, inexpensive
Examine multiple effects of a single exposure
Minimizes bias in the in exposure determination
Direct measurements of incidence of the disease
Validity can be affected by losses to follow-up
Case-control study
Quick, inexpensive
Well-suited to the evaluation of diseases with long latency period
Rare diseases
Examine multiple etiologic factors for a single disease
Selection Bias and recall bias

29. Key points in Cohort study
Presence or absence of risk factor is determined before outcome occurs
Identify cohort (s).
Measure exposure and outcome variables
Follow for development of outcomes
Estimate incidence rates, RR and AR, if possible population AR.

30. Motivation is what gets you started. Habit is what keeps you going.
Thought for the day
Motivation is what gets you started.
Habit is what keeps you going.
-Jim Ryun

31. Thank you

34. Strengths of Cohort Studies
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

35. 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

36. Cohort study Example of cohort studies
 The association between statin use and prostate cancer risk
 All men 45+ years enrolled in GHC for at least 2 years during
 Exposure is statin use, which may change over 14 years
 Follow 14 years until develop prostate cancer, die, or disenroll from GHC
 Each subject will contribute person-time to follow-up  Survival analysis to account for time varying exposure, adjust for other risk factors, & account
for censoring
Prostate cancer
No cancer
Statin user
Nonuser
14 years

37. Prospective vs. retrospective Cohort Studies
Prospective Cohort Studies
Time consuming, expensive
More valid information on exposure
Measurements on potential confounders
Retrospective Cohort Studies
Quick, cheap
Appropriate to examine outcome with long latency periods
Admission to exposure data
Difficult to obtain information of exposure
Risk of confounding
The prospective study is time consuming and therefore expensive on the other hand it is possible to have valid information on exposure, because it isn.t

38. Analysis in Cohort study
 Exposed and non-exposed individuals are
followed over time to determine whether they
experience the outcome of interest
 Examples of exposure:
Environmental factor, condition, procedure
 Examples of outcome:
Disease, death, costs