Overview of Cohort Study design - characteristics, type, guide to conducting a study, advantages, disadvantages and potential sources of bias

1. Cohort Study Design
Dr K.A Afolabi

2. Outline
 Overview
 Steps in conducting a cohort study
 Advantages
 Disadvantages
 Potential sources of bias

4. Overview
 Also referred to as Prospective, Incidence, Longitudinal
or Follow up studies
 Exposure status is determined amongst a group of
persons sharing the same experience (cohorts) followed
for a specified period of time to determine the incidence
of a disease or event (outcome)
 Comparison of incidences to test causative/protective
hypothesis
 Example:
 Framingham Study: Risk factors in 30 -62 yr old
residents(cohorts)  CHD (Outcome)
 Dolls: In Physicians of same age group (Cohorts)
Smoking (Exposure)  Lung cancer (Outcome)

6. Typesof Cohort
Studies
Prospective/Concurren
t
Retrospective/Historical

7. StepsinaCohort
Study:
Selection ofstudy
population
Depends on:
 Frequency of exposures of interest
 for exposures common among the general population -
the general population
 Groups of individuals with special/unusual exposures.
E.g. occupational hazards, Folic acid supplementation in
pregnancy
 Need complete and accurate exposure and follow-up
information

8. StepsinaCohort
Study:
Selection of
Exposed/Unexpos
edgroups
 Clear definition
 The definition must include the following:
 The minimal acceptable levels of exposure
 The minimal duration of exposure
 Other eligibility criteria like age, sex, absence or
presence of pre-existing medical conditions
 the exclusion criteria must include the absence of the
outcome of interest
 Data may be collected from pre-existing records,
directly from interviewing the subjects, measurements
etc.

9. StepsinaCohort
Study:
Determining
Outcome
Present/Absent
Groups
 Duration of follow-up to determine outcome dependent
on disease latency period
 Clear definition - process of establishing outcome must
be clearly stated
 Data may be collected from Routine surveillance data,
Death certificates, Examination of the cohorts, Records
etc.

11. Advantagesof
CohortStudies
 Measures incidence
 Gives a direct measurement of risk
 Dose effect can be determined
 Temporal relationship between exposure & disease is
clear
 Minimizes selection and information bias (especially
Prospective cohort studies)
 Well suited to rare exposures
 Several outcomes can be examined in one study

12. Disadvantages
ofCohort
Studies
 Often requires large sample size
 Latency period: long follow-up period or bias
 Attrition - loss to follow-up can affect validity of
findings
 Exposure can change over time
 Ineffective for rare diseases
 Difficult to assess multiple exposures
 Time consuming and expensive
 Some problems of bias may occur
 Ethical issues

13. PotentialBiases
inCohort
Studies
 Selection bias
– loss to follow-up
 Information bias
– from different quality and extent of
information obtained
 apply the same protocol for measuring or evaluating the
health outcomes in exposed and nonexposed individuals
 Blinding

14. Thank you for your time