Cohort studyCohort study
Dr K N Prasad MD., DNBCommunity Medicine
Aim of Epidemiological studies
1. To determine distribution of disease
2. To examine determinants of a disease
3. To judge whether a given exposure causes or prevents disease
Epidemiological study designs1. Descriptive studies
1. Populations 1. Correlated studies
2. Individuals1. E.g. case-series, case reports, cross-sectional surveys
2. Analytical studies1. Observational studies
1. Case-control studies2. Cohort studies
3. Experimental studies 1. Intervention studies after randomise exposure
1. Clinical trials
Case-control study
Study Population
Cases
Controls
Exposed
Non-exposed
Exposed
Non-exposed
Cohort study / Follow-up study
Study population
Exposed
Non-exposed
Disease +
Disease +
Disease -
Disease -
General considerationsA cohort :A cohort :
A group of persons, identified at one point in time, who march off together into the future under the watchful eye of an investigator.
A cohort study:
A group of persons is defined, certain characteristics about each individual are recorded, and they are then followed up in such a way that new events (such as disease and death) or other changes in their characteristics are detected.
Cohort StudyCohort Study Longitudinal study, Follow-up study,
prospective study
DefinitionDefinition: An analytical epidemiological study in which two or more groups of people according to the extent of exposure (e.g. exposed and unexposed) are compared with respect to outcome or disease incidence
Most reliable for showing an association between a suspected risk factor and subsequent disease
Features of cohort study
1. Cohorts must be free from the disease under study
2. Both the groups should be equally susceptible to disease under study
3. Diagnostic and eligibility criteria of the disease must be defined beforehand
Cohort studyCohort study Exposed and non exposed individuals are
followed over time to determine whether they experience the outcome of interest.
Examples of exposureExamples of exposure : Medication use, Environmental factors,
condition, Procedure
Examples of outcomeExamples of outcome: Disease. Death, etc.
Cohort studies• Retrospective
– Exposure Disease• Yes ?• No ?
• Prospective– Exposure Disease
• Yes ?• No ?
• Ambidirectional
Timing of cohort studies
• Retrospective: both exposure and disease have occurred at start of study
Exposure------------------------Disease *Study starts
Timing of cohort studies
• Prospective: exposure has (probably) occurred, disease has not occurred
Exposure----------------------Disease *Study starts
• Ambi-directional: elements of both
Elements of cohort studyElements of cohort study
1. Selection of study subjects( cohorts)2. Selection of comparison group3. Obtaining data on exposure4. Follow up5. Analysis
Selection of the Exposed Population
Sample of the general population:Geographically area, special age groups, birth
cohorts (Framingham Study)
A group that is easy to identify:Nurses health study
Special population (often occupational epidemiology):Rare and special exposurePermits the evaluation of rare outcomes
Selection of the Comparison Selection of the Comparison PopulationPopulation
Internal Control GroupExposed and non-exposed in the same Study
population (Framingham study, Nurses health study)Minimise the differences between exposed and non-
exposed
External Control GroupChosen in another group, another cohort
(Occupational epidemiology: Asbestosis vs. cotton workers)
The General Population
Selection of comparison groupSelection of comparison groupInternal comparison group: according to the
degrees or levels of exposureSmokers, BP, Alcohol, diet etc.
External comparisonSimilar in all respects without any exposure
Comparison with general population ratesOutcomes are compared with the similar
outcome rates in the general population
Sources of exposure information:Sources of exposure information:
Pre-existing records - inexpensive, data recorded before disease occurrence but level of detail may be inadequate.
Records may be missing, / usually don't contain information on confounders
Sources of exposure information:Sources of exposure information:
Questionnaires, interviews: good for information not routinely recorded but
have potential for recall bias
Direct physical exams, tests, environmental monitoring may be needed to ascertain certain exposures.
Follow upFollow upRegular follow up of all participants
Periodic medical examination of each memberReviewing physician and hospital recordsRoutine surveillance of death recordsMailed questionnaires, telephone call, periodic
home visits
Sources of outcome information:Sources of outcome information:
Death certificatesPhysician, hospital, health plan recordsQuestionnaires (verify by records)Medical examinations
Analysis in cohort studyAnalysis in cohort study
1. Incidence of disease among exposed and non exposed
2. Relative risk estimation
3. Attributable risk estimation
Table for analysis cohort studyTable for analysis cohort studyDisease present
Disease absent
Total
Exposure Present ( cohort)
a b a + b
Exposure absent (comparison)
c d c + d
Total a + c b + d a+b+c+d
Incidence of disease among exposed = a / a+b
Incidence of disease among non exposed = c / c+d
P value should be <0.05
Relative riskRelative risk
Relative risk is calculated as
Incidence of disease among exposedIncidence of disease among non exposed
Relative risk (Risk ratio)Relative risk (Risk ratio)Quantifies magnitude of the association between
exposure and disease
Varies from 0 to infinity RR<1: exposure decreases the risk for disease RR=1: no association RR>1: exposure is a risk factor for disease;
increases risk for disease
Example: RR=2.0 can be interpreted as two fold increase in
risk
Attributable risksAttributable risks1. Also known as risk difference2. It is the difference in incidence rates of disease
between exposed group and non exposed group.3. It suggests the amount of disease that might be
eliminated if the risk factor could eliminated or controlled.
Incidence of disease among exposed - incidence of disease among non exposed
------------------------------------------------------- x 100Incidence of disease among non exposed ex. AR is 90%. Interpretation-
Bias in Cohort studyBias in Cohort study1. Selection bias - less of a problem than case
control studies2. Information bias/misclassification3. Degree of accuracy of classification of
exposure, confounders and disease status4. Loss to follow-up (affects validity)5. Non response (limits generalisability, not
validity)6. Confounding
Cohort studyCohort studyLimitationsLimitations Loss to follow-up Misclassification of diseaseor exposure status logistically challenging –
especially for prospective design
Hard to study rare diseases Changes over time in
staff/methods Little control over natureand quality of data inretrospective designs
StrengthsStrengths Can establish time order Can obtain incidence rates Can study more than onedisease or outcome Minimizes bias inascertainment of exposurestatus and covariates –especially if collecting dataprospectively Efficient for rare exposures No controls, so no bias incontrol selection
Cohort study• Rare exposure• Examine multiple
effects of a single exposure
• Minimizes bias in the in exposure determination
• Direct measurements of incidence of the disease
• Validity can be affected by losses to follow-up
Case-control study• Quick, inexpensive• Well-suited to the
evaluation of diseases with long latency period
• Rare diseases• Examine multiple
etiologic factors for a single disease
• Selection Bias and recall bias
Key points in Cohort studyKey points in Cohort study1. Presence or absence of risk factor is
determined before outcome occurs2. Identify cohort (s).3. Measure exposure and outcome
variables4. Follow for development of outcomes5. Estimate incidence rates, RR and AR, if
possible population AR.
Thought for the dayThought for the day
Motivation is what gets you started.Habit is what keeps you going.
-Jim Ryun
Thank youThank you
Strengths of Cohort Studies
• Efficient for rare exposures, diseases with long induction and latent period
• Can evaluate multiple effects of an exposure
• If prospective, good information on exposures, less vulnerable to bias, and clear temporal relationship between exposure and disease
Weaknesses of Cohort Studies
• Inefficient for rare outcomes
• If retrospective, poor information on exposure and other key variables, more vulnerable to bias
• If prospective, expensive and time consuming, inefficient for diseases with long induction and latent period
• Keep these strengths and weaknesses in mind for comparison with case-control studies
Cohort study• Example of cohort studies• The association between statin use and prostate cancer risk• All men 45+ years enrolled in GHC for at least 2 years during 1990-2004• Exposure is statin use, which may change over 14 years• Follow 14 years until develop prostate cancer, die, or disenroll from GHC• Each subject will contribute person-time to follow-up Survival analysis
to account for time varying exposure, adjust for other risk factors, & account• for censoring • Prostate cancer• No cancer• Statin user• Nonuser• 14 years• Prostate cancer• No cancer
Prospective vs. retrospective Prospective vs. retrospective Cohort StudiesCohort Studies
• Prospective Cohort Studies– Time consuming, expensive– More valid information on exposure– Measurements on potential confounders
• Retrospective Cohort Studies– Quick, cheap– Appropriate to examine outcome with long latency
periods– Admission to exposure data– Difficult to obtain information of exposure– Risk of confounding
Analysis in Cohort studyAnalysis in Cohort study Exposed and non-exposed individuals arefollowed over time to determine whether theyexperience the outcome of interest
Examples of exposure:Environmental factor, condition, procedure
Examples of outcome: Disease, death, costs