2. Outline
Overview
Steps in conducting a cohort study
Advantages
Disadvantages
Potential sources of bias
3.
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)
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.
10.
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
The cohorts are similar in all other experiences/factors except the exposure of interest.
Open/Dynamic cohort – is one where people can enter or leave. Examples: A workforce study that is ongoing, A city or other geographic location. A closed cohort is where all persons in the cohort are defined at entry e.g UI medical school class of 2004
Special group exposures: exposure to aniline (dye factory workers), Use of PPE or not (health workers)
For accurate information, purposely selecting groups likely to give accurate and complete information or groups that have available record of the exposure
RR=1 No association
RR>1 Risk of outcome increased with exposure (causative)
RR<1 Risk of outcome decreased (protective factor)
Information bias: mostly from ‘over-searching’ for outcome in exposed
Prevented by applying same protocol for both groups and by masking exposure status from the person who decides presence or absence of outcome
Health worker effect
Hawthorne effect – exposed changing to unexposed withou