Introduction to Cohort Studies

MalimuKampala international University

Learning Objectives

When you have completed this session you will be able to:

Describe the characteristics of a cohort study List the types of bias most likely to affect a cohort

study List the conditions under which a cohort study is an

appropriate choice to address a research question Describe the advantages and disadvantages of

cohort study

PersonPlace

Time

Cases

0

5

10

15

20

25

1 2 3 4 5 6 7 8 9 10

0200400600800

10001200

0-4 '5-14 '15-44

'45-64

'64+

Age Group

Descriptive Epidemiology

Who? Where? When?

But sometimes also: Why? What are the risk factors for neonatal

tetanus? What factors are associated with increased

mortality for persons with measles? Does smoking cause lung cancer?

Analytical epidemiology

Analytic Studies

To test whether certain factors are “associated” Association is a statistical concept To look at association we need to move away from

description of a factor in one group We need a “comparison group” Is cancer more common in those exposed to uranium

compared to those who have never being exposed ? Is uranium associated with cancer ? Is this association statistically significant ? Could it have occurred by chance ?

Case-control

Cohort

Individuals

InterventionRetrospective

Prospective

Descriptive

Populations

Analytical

Observational

Case-series

Cross-sectional

Correlational

Clinical trials

Epidemiological studies

Observational Analytic Studies

exposure outcome cohort

case-control

cross-sectional

exposure outcome

exposure outcome

Disease among exposed?

Disease among non-exposed?

Usually prospective

Cohort study

Populationat risk

Exposed

Not Exposed

and

What is a cohort?

Cohort: Latin word for 1 of the 10 divisions of a Roman legion

A group of individuals sharing same experience Followed-up for a specified period of time

Examples birth cohort Workers at a chemical plant KIU first cohort

Design of a Cohort Study

Individuals “choose” theirexposure status

Sub-classifications of Cohort Studies Time perspective

Prospective Retrospective

Population dynamics Closed population Open population

Terminology: Retrospective or Prospective?

Suggest use the terms “retrospective” or “prospective” to refer to the timing of events in relation to initiation of study.

(Hennekens and Buring, 1987)

Retrospective cohort study: exposure and disease have occurred prior to start of study

Prospective cohort study: disease has not occurred prior to start of study

Exposureoccurrence

Study starts Diseaseoccurrence

Prospective Cohort Study

Time

+-

+ -ill

exp+-

exp

Prospective assessment of exposure and disease

Selection of population

Prospective Cohort Study

Chernobyl, Industrial accidents, Flood victims

+-

+ -ill

exp+-

exp

Diseaseoccurrence

Study startsExposureoccurrence

Prospective assessment of disease

Selection based on exposure

Steps in a prospective cohort study

Define the population at risk (=cohort) Determine exposure to a factor of interest of all

subjects in the cohort Follow exposed and non-exposed forward in time to

ascertain whether they develop the outcome of interest Compare the outcomes in the exposed and the

unexposed group with each other

Foodborne outbreaks, closed environment outbreaks (school, prisons, …)

Retrospective Cohort Study

Study takes place

Diseaseoccurrence

Exposureoccurrence

Retrospective assessment of exposure and disease

Selection based on population

+-

+ -ill

exp

Retrospective cohort studies

• Well defined population • Exposed and unexposed can be identified • Outcome (ill or not ill) can be ascertained Opportunity to go back in time, categorise people according to their exposure and then determine their outcome

For example, weddings, parties, hotels, occupational exposures, etc.

Closed and Open Populations

Closed population adds no new members over time loses members only to death

Open population may gain members over time (immigration or

birth) may lose members who are still alive through

emigration

Issues in Design of Cohort StudiesSources of DataExposure Information Pre-existing records

Availability for much of cohort Inexpensive Objective, bias-free categorisation of exposure status But – insufficient detail and no information on potential confounders

Information from study subjects Information on data not routinely collected Questionnaires/interviews Potential bias

Ascertainment of exposure must be comparable for all

Issues in Design of Cohort StudiesSources of DataOutcome Information Obtain complete, comparable, unbiased information Death certificates (potential bias when cause-

specific mortality) Medical records, Medical Aid schemes, etc. From study subjects Periodic direct medical examinations

Apply equally to exposed and non-exposed

Biases in Cohort studies

1. Loss to follow up Failure to ascertain outcome data is the major

source of potential bias Length of follow-up period is related to latency

period of disease The longer the follow-up period the more difficult

to ensure complete data If lost to follow-up is large (eg, 30-40%) ?

Validity ? Loss to follow-up may be differential

2. Participation bias

Agreeing participants may differ from non-participants

This affect external validity more then internal3. Misclassification bias Misclassification due to exposure status is

common Can be random (equally for exposed and

unexposed) or non-random

4. Ascertainment bias

Biases in ascertaining the outcome. Outcomes should be ascertained equally by

exposure status

Distribution of illness according to exposure in a cohort study

Exposed

Not exposed

ILL NOT ILL

a b

c d

a+b

c+d

Incidence

a+b

c+d

a

c

Relative risk = Incidence exposed / Incidence not exposed

Cohort study about bottled water as risk factor for illness

Drink bottled water 40

30

30

Risk

3040

Relative Risk (RR) = 30 / 405 / 30

= 4.4

ILL NOT ILL

10

25 5Do not drinkbottled water

5 30

Interpretation of Relative Risk The risk of illness among those who drink

bottled water is 4.4 times higher than among those who do not drink bottled water.

Advantages of cohort studies Directly measure risk or rate Measures of effect have clear meaning and are

easily understandable Temporal relationship between exposure &

disease can be established Prospective cohort studies less susceptible to

selection bias because outcome not known Well suited to rare exposures Several outcomes can be examined in one study

Disadvantages of cohort studies

Large sample size Latency period Loss to follow up Exposure can change over time Multiple exposures = difficult Cost Time consuming

Recap

Now that you have completed this session you should be able to:

Describe the characteristics of a cohort study List the types of bias most likely to affect a cohort

study List the conditions under which a cohort study is an

appropriate choice to address a research question Describe the advantages and disadvantages of

cohort study versus a case-control study