A cohort study examines the effect of exposures on a group of subjects over time by collecting data on exposures and outcomes. It has several advantages: the temporal sequence between exposure and outcome provides evidence of causality, multiple outcomes can be examined, and rare exposures can be studied. However, cohort studies are costly, prone to dropout, and require large sample sizes when studying rare outcomes. They also cannot prove causality definitively.

1. KNOWLEDGE FOR THE BENEFIT OF HUMANITYKNOWLEDGE FOR THE BENEFIT OF HUMANITY
PUBLIC HEALTH AND EPIDEMIOLOGY (HFS3063)
Epidemiological Study Designs:
COHORT STUDY
Dr.Dr. MohdMohd RazifRazif ShahrilShahril
School of Nutrition & DieteticsSchool of Nutrition & Dietetics
Faculty of Health SciencesFaculty of Health Sciences
UniversitiUniversiti SultanSultan ZainalZainal AbidinAbidin
1

2. Topic Learning Outcomes
By the end of this lecture, students should be able to;
• describe cohort study design.
• explain the advantages and disadvantages of cohort
study design.
2

3. Cohort studies
• A group of subjects, selected to represent the population
of interest, is studied over time.
• Information is collected about the outcome of interest
and exposure to risk factors.
• Subjects are disease-free at the outset of the study.
• At distinct point of time, data are collected relating to
health outcomes and exposure to risk factors.
• Used to examine causal factors.
3

4. Prospective vs. Retrospective Cohort
• Prospective cohort studies
– Studies carried out from present time into future.
– Can be tailored to collect specific exposure data.
– But long wait for events to occur.
– Expensive.
– Prone to high dropout rates.
• Retrospective (historical) cohort studies
– Look at medical events from some timepoint in the past up to the
present time.
– Advantage: Information available immediately.
– Disadvantage: (1) difficulty in tracing subjects, (2) reliance on the
memory of subjects and/or the quality of recorded information
4

5. (cont.) Prospective vs. Retrospective Cohort
5

6. Selection and follow-up of subjects
• The ‘healthy entrant effect’
– a disease free status on entry to the study
– Initially, subjects are seen to have lower levels of disease than
might be true of the population in general
• with an acceleration of disease rate over time
• Following cohorts from birth may overcome ‘healthy
entrant effects’
– May result in lengthy and costly study
• Follow-up of subjects is carried out to monitor changes in
health status over time.
• Loss to follow-up will increase with the length of study.
– Plan mechanism to achieve lowest possible dropout rate.
6

7. (cont.) Selection and follow-up of subjects
• Systematic differences between those who drop out
and those who stay in the study
– related to the outcome or exposure to risk factors
• Analysis of data must include a comparison of risk
factors between individuals who remain in the study and
those who have dropped out.
• If loss to follow-up is ignored
– Reliability of study conclusion can be questioned.
7

8. Inferring causality
• To infer causality with any degree of certainty
– experimental study design
• Longitudinal nature of cohort studies
– Enables the assessment of causal hypotheses
– It is known if exposure occurred prior to outcome
• Measuring changes in levels of exposure over time
alongside changes in outcome measure gives
information on
– Dose response relationship between exposure and outcome .
• Higher level of exposure, associated with higher levels of
outcome
– provide further argument for causality.
8

9. Hypotheses of cohort studies
Straightforward comparison of two groups, those with
exposure and those without.
• Ho: on average, the disease in the exposed group is no
different to that of the unexposed group
• E.g.: comparing cancer occurrence among those who
smoke with those who do not.
9

10. (cont.) Hypotheses of cohort studies
When there are more than two groups.
• E.g.: The smoking status are split into those who do not
smoke, those who smoke fewer than 20 cigarettes a day
and those who smoke over 20 cigarettes.
• Type of grouping in strata, it has three hypotheses.
– Ho: on average, the outcome rates in groups of subjects are no
different to that of the population as a whole
– Ho: on average, the outcome rates in groups of subjects are no
different from each other
– Ho: on average, there is a linear trend in outcome rates across
groups
10

11. Analysis of data
• Statistics from cohort study;
– Crude rates of outcome
– Standardized rates and ratios of outcome
– Risk ratio of outcome
• Crude Rates
– Number of individuals with the outcome out of the total cohort
study size
11
(a + b) / n

12. (cont.) Analysis of data
• Standardized Rates and Ratios
– Can be calculated using demographic information so that rates
of the outcome are adjusted for other potential risk factors such
as age and sex
• Risk Ratio
– The risk of the outcome in exposed subjects relative to those not
exposed
12
[a / (a + c)] / [ b / b + d)]

13. Advantages of cohort studies
• The temporal dimension, whereby exposure is seen to
occur before outcome, gives some indication of
causality.
• Can be used to study more than one outcome.
• Good for the study of rare exposures.
• Can measure the change in exposure and outcome over
time.
• Incidence of outcome can be measured.
13

14. Disadvantages of cohort studies
• Costly (less so for retrospective) and may take long time
– Particularly where onset of the outcome measure can occur both
early and late on in life.
• Require accurate records for retrospective studies.
• When studying rare outcomes, a very large sample size
is required.
• Prone to dropout.
• Changes in etiology of disease over time may be hard to
disentangle from changes observed as age increases.
• Selection bias
– A difference in incidence of the outcome of interest, between
those who participated and those who did not, would give biased
results.
14

15. Recapitulate
In this lecture, you have been exposed to;
• definition and types of cohort study .
• selection and follow-up of subjects.
• inferring causality
• hypotheses of cohort studies.
• analysis of data.
• advantages of cohort studies.
• disadvantages of cohort studies.
15

16. Thank YouThank You
16