The document outlines the cohort study design, including its overview, advantages, disadvantages, and potential sources of bias. It details the steps in conducting a cohort study, such as selection of the study population, defining exposure groups, and determining outcomes. Examples illustrate the approach, emphasizing the importance of proper methodology to minimize bias and accurately assess health outcomes.

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