This document discusses cohort studies. A cohort study compares outcomes between groups that differ in their exposure to a risk factor. It involves selecting groups of individuals, measuring their exposure to a risk factor, observing them for a defined outcome, and analyzing any association. The key elements are defining the study question, selecting and measuring exposure in study populations, following up to ascertain outcomes, and analyzing results like incidence rates and relative risks. Cohort studies provide strong evidence but require large sample sizes and long follow-up periods.

1. Dr Sridevi NH
MBBS (MD)
Community medicine
chikmagalur

2. Contents
 Introduction
 Hierarchy of scientific evidence
 Indications for cohort study
 Elements of a cohort study
 Types
 Advantages and disadvantages
 Summary
 References

3. Introduction
 Cohort study is a type of analytical study.
 Cohort is defined as a group of people who share a
common characteristic or experience within a
defined time period.

4. what is cohort study???
 Compare - occurrence of events /outcomes among
groups of people whose exposure status differs
‘naturally’.

7. Indications for cohort study
 There is good evidence of association between
exposure and disease.
 Exposure is rare, but the incidence of disease
among exposed is high.
 When attrition of study population can be minimized.
 When ample of funds are available.

8. Elements of cohort study
 Defining the study question
 Selecting the study population
 Measure exposure
 Follow up of participants
 Ascertaining outcome
 Analysis and results

9. Defining the study question
 A study question is framed.
 Important terms should be defined.
 The diagnostic and eligibility criteria of the disease
must be defined beforehand.

10. Selecting the study population
Selection of study subjects.
Selection of comparison groups.
 Internal comparison ---Common exposure Ex: smoking, tobacco
chewing etc
 External comparison---rare exposure Ex: Industrial chemicals (
different industries), Ionizing radiation
 Comparison with general population.
General
population
Special
groups
Select
groups
Exposure
groups

11. Formation of cohort
Cohorts/ subjects selected must be:-
Free from Outcome under study.
Must be equally susceptible for the outcome.
Both the groups should be comparable in respect of
all the possible variables, which may influence the
frequency of outcome.
Must be amenable for follow-up with least possible
attrition.

12. Healthy worker effect
 When exposed group is working population and
Comparison group is general population.
 Working population is healthier
 General population includes- Sick to work.
How to reduce???
• Same workforce – comparison group with different
level of exposure
• Comparison group in different industry

13.  External comparison group is general population
 people with unhealthy lifestyle refuse to participate
 Volunteers as comparison group.
What if not convinced with comparison group????
 Two or more comparison group
 Industrial chemical and cancer
 Exposed is – workers in industry
 Unexposed – General population, another industry.

14. Steps
 Defining the study question
 Selecting the study population
 Measure exposure
 Follow up of participants
 Ascertaining outcome
 Analysis and results

15. Measure exposure
 Interviewing cohort members
 Medical records ,Employment records
 Medical examination/diagnostic procedures.
 Environmental surveys.

16.  Effect of Ionization radiation on workers
Exposed are workers in the unit.
Unexposed are – those residing in vicinity but not working
in unit
Unexposed group is genuinely unexposed to exposure
 Accurate measurement- to avoid Misclassification
 Exposure status may change during follow up period
 Reassess at periodic interval

17. Information bias
 Inadequacy of information derived from medical or
other records.
 Difference in Quality and extent of information
obtained for exposed and non exposed groups
 Limited sensitivity and specificity of the diagnostic
tests involved
 Applying tests of differing sensitivities, differentially
to study groups

18. Steps
 Defining the study question
 Selecting the study population
 Measure exposure
 Follow up of participants
 Ascertaining outcome
 Analysis and results

19. Follow up of participants
 Periodic medical examination of each member of the
cohort.
 Reviewing physician and hospital records
 Routine surveillance of death records
 Mailed questionnaires, telephone calls, periodic
home visits –preferably all three on an annual basis
 Potential weakness.

20. Biases
 Lost to follow up due to
 Death
 change of residence
 Migration
 withdrawal of occupation

21. How to reduce??
 Ensure completeness of follow up – develop
mechanism for that.
 Special group of population can be selected.
 Contact relatives to collect information.

22. Steps
 Defining the study question
 Selecting the study population
 Measure exposure
 Follow up of participants
 Ascertaining outcome
 Analysis and results

23. Ascertaining outcome
 Records:-
 Uniformity
 completeness
 method adopted may differ between time periods.
 Interviews/questionnaires:-
 Validity
 reliability
 social desirability.
 Direct measurements:-
 variations
 measurement error.

24. Sources of data for outcomes
 Outcome fatal – death certificates
 Routine surveillance
 Autopsies
 Medical Records
 Registries
 Insurance data
 Periodic direct medical examination

25. Bias in assessment of the
outcome
If the investigator who assess outcome is aware of
exposure status of subjects, then the judgment as to
whether the disease developed may be biased by
that knowledge.
 Masking the investigator regarding exposure status.
 Newer modalities of outcome ascertainment

26. Steps
 Defining the study question
 Selecting the study population
 Measure exposure
 Follow up of participants
 Ascertaining outcome
 Analysis and results

27. Analysis and results
The data is analyzed in terms of :-
1. Incidence rates of outcome among exposed and
non exposed
2. Estimation of risk
 relative risk
 attributable risk

28. Incidence rates
 Estimation of disease incidence rates (λ)
 Calculate total number of events observed among all
individuals (D)
 Calculate the sum of the individual observation times (Y)
– person years at risk
 Calculate λ = D/Y  incidence rates
 Rate ratio = λ1/ λ0
 Rate difference = λ1- λ0

29. Individual’s observation time
 starts when subject joins the study
 stops when
 – Subject develops the disease
 – Subject is lost to follow-up
 – The follow-up period ends

31. Relative risk
 RR= incidence of disease among exposed
incidence of disease among non exposed
It is the direct measure of the strength of association
between suspected cause and effect.
RR=1 indicates no association
RR>1 indicates positive association
RR<1 indicates negative association

32. Attributable risk
Absolute difference between the two risk estimates. It estimates the
absolute excess risk associated with a given exposure

33.  Percent Attributable Risk= Iexp - Inon exp x 100
I exp
 AR% = (RR –1) / (RR) x 100

34. Population attributable risk
 1953- Levin - first described this measure (smoking
& lung Ca)
 It gives the measure of disease risk in the total
population associated with the exposure
 AR= Iexp – Inonexp
 PAR= Ipop – Inonexp

36. PAR estimates the amount by which the disease
could be reduced in the population if suspected
factor is eliminated or modified.

37. Types of cohort study
 Prospective cohort study
 Retrospective cohort study
 Retro-Prospective cohort study
 case cohort study

38. Prospective cohort study
 More expensive
 Requires long time follow up

39. Retrospective cohort study
 Here outcomes would have occurred before the start
of the investigations
 The investigator goes back in time, select the study
groups from the records, and traces them forward
through time up to the present.
 Faster, less expensive
 Useful for disease with long period between
exposure and outcome
 Data on exposure and confounders – may be less
accurate

40. Retro prospective study
 Both the retrospective and prospective elements are
combined.
 Cohort is identified from past records, and is
followed up prospectively for the assessment of
outcome

42. Case cohort study

43. Advantages
 Incidence can be calculated
 Several possible outcomes related to exposure can
be studied simultaneously.
 Provide a direct estimate of relative risk
 Dose response ratio can also be calculated.
 Bias can be minimised
 PAR can be calculated.

44. Disadvantages
 Involve large number of people- unsuitable for
investigating uncommon diseases
 It takes long time to complete study and obtain results
 Administrative problems are inevitable
 Lost to follow up
 It is difficult to introduce new knowledge and
technologies.
 Expensive
 Ethical issues.
 Study itself may alter peoples behavior.

45. Summary
 Cohort study constitutes the design of reference for all
other designs in observational epidemiology. The
advantage of cohort design is the possibility of studying
the occurrence of several events in addition to that
initially planned.
 Cohort studies have proven to be extremely valuable in
many areas of medical research including cardiovascular
disease, cancer, infectious diseases. Data from these
studies is of great use for the development of public
policy, such as treatment guidelines.

46. References
1. Park K. Cohort study . Parks text book of
Preventive and social medicine. 23rd edition.
Jabalpur, M/S Banarsidas Bhanot Publishers.
January 2015 page 75-80
2. Leon Gordis. Cohort studies. Epidemiology 4th
edition. Elsevier. 2009 page 167-176.
3. Roger detels. Oxford textbook of public health. 5th
edition. Oxford 2009 page 508-525
4. Textbook of public health and community medicine
1st edition 2009. AFMC page 147-150