Cohort study design

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
Exposurestatus 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 yrold
residents(cohorts)  CHD (Outcome)
Dolls: In Physicians of same age group (Cohorts)
Smoking (Exposure)  Lung cancer (Outcome)

6. Types of Cohort
Studies
Prospective/Concurren
t
Retrospective/Historical

7. Steps in a Cohort
Study:
Selection of study
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. Steps in a Cohort
Study:
Selection of
Exposed/Unexpos
ed groups
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. Steps in a Cohort
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. Advantages of
Cohort Studies
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
of Cohort
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. Potential Biases
in Cohort
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