Cohort Study Designs

Cohort Study Designs

Ahmed MandilDept of Family & Community Medicine

College of MedicineKing Saud University

Headlines

Definitions Observational studies Cohort studies Advantages, disadvantages Examples Analysis

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Observation Methods

Selected Units: individuals, groups Study Populations: cross-sectional,

longitudinal Data collection timing:

prospectively, retrospectively, combination

Data collection types: primary, secondary

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Study populations Cross-sectional: where only ONE set of

observations is collected for every unit in the study, at a certain point in time, disregarding the length of time of the study as a whole

Longitudinal: where TWO or MORE sets of observations are collected for every unit in the study, i.e. follow-up is involved in order to allow monitoring of a certain population (cohort) over a specified period of time. Such populations are AT RISK (disease-free) at the start of the study.

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Study Designs in Health Research

Qualitative Focus group

discussions Key informant

studies Ethnographic

studies Bibliographic

studies Others

Quantitative Observational

Descriptive, e.g. cross-sectional

Analytical, e.g. case-control; cohort

Experimental (also analytical)

Randomized Clinical Trials

Community Intervention Studies


Cohort Studies (I)

(a) Characteristics: A “cohort” is a group of people, referred to as

“disease-free population” or “population at risk” A survey is first carried out to exclude prevalent

cases from the cohort A period of "follow-up“ is specified, for possible

new cases' occurrence We know the exposure status, looking for the

disease status Historical designs are preferred under

occupational settings, for less frequent effects / exposures.


Cohort Studies (II)

Two types are recognized: Prospective (longitudinal):

forward in time follow-up study Retrospective (historical):

backward in time study (depends on records: medical / employment). This is the type preferred under occupational settings


Coh

ort

Desi

gn

time

Study begins here

Studypopulation

free ofdisease

Factorpresent

Factorabsent

disease

no disease

disease

no disease

presentfuture


Cohort Studies (III)

(b) Advantages: No / little temporal ambiguity (suggests cause-

effect relationship) Calculation of incidence rates Suitable for rare exposures Factors associated with selection cannot

influence disease status and hence the results. Several outcomes can be studied, after follow-

up starts.


Cohort Studies (IV)

( c ) Disadvantages (of prospective):

Expensive Time-consuming May be impractical Loss to follow-up may affect

sample-size


IDEAL COHORT

An ideal cohort should be: STABLE COOPERATIVE COMMITTED WELL-INFORMED


Examples of Study Cohorts

1. General population

2. Selected occupational groups, e.g. health professionals (physicians, nurses, lab technologists, etc), manufacturers of mercury batteries for vehicles; asbestos workers, miners, etc04/20/23 12Cohort Studies

Sources of Cohorts

Population groups Occupational settings

(employment, medical records) Hospital registers (medical

records) Death certificates


Follow-up Techniques

1. Periodical medical examinations and mailed questionnaires

2. Direct personal interviews or examinations 3. Videoconference, neighbors, friends and relatives4. Lost persons can be traced through the letters, from their relatives and friends5. Migrated cohort subjects can also be traced through

travel and immigration authorities6. Dead persons - local or regional mortality registers or death certificates


Problems during Follow-up

FOLLOW-UP OF A LARGE GROUP LIMITED RESOURCES TIME SCARCITY PAUCITY OF TRAINED PERSONNEL ATTRITION, LOSS ON FOLLOW UP ETHICAL CONCERNS


Attrition Reduction

OBTAINING THE INFORMED CONSENT RECORDING COMMITMENT TO CONTINUE AND

COOPERATE IN THE STUDY TRACING LOST SUBJECTS, TRYING TO INCLUDE

THEM IN THE STUDY CONSIDERING INFORMATION OF LOST PERSONS

AT THE TIME OF ANALYSIS KEEPING NON-RESPONSE AT A LOW LEVEL TO

IMPROVE THE VALIDITY


Examples of Cohort Studies

1. POPULATION-BASED

1. CARDIOVASCULAR

2. CHILD HEALTH

3. SPECIAL EXPOSURES

2. NON-POPULATION BASED

1. OCCUPATIONAL – for convenience

2. OCCUPATIONAL – to study the occupation

3. HEALTH CARE SETTINGS

4. VETERANS


CARDIOVASCULAR DISEASE

USA: Framingham, MA; Tecumseh, MI; Evans county, GA; Muscatine, IA; Bogalusa, LA (children)

WHO MONICA (multi-center) North Karelia, Norway


Framingham Study (1951 – present time)

1ST Step: Selection of cohorts Initially, 5209 subjects were

enrolled into the study Currently, thousands of people are

followed up, both for cardiovascular risk factors (e.g. high serum cholesterol, smoking, hypertension, BMI, etc) and possible outcomes


SPECIAL EXPOSURES Atomic Bomb Casualty Commission

(ABCC): Hiroshima and Nagasaki survivors (effects of radiation)

Dutch famine survivors (effects of starvation)

Seveso (effects of dioxin exposure)


OCCUPATION BASED, TO STUDY EXPOSURES

Benzene-workers (leukemia) Coke-oven workers (lung

cancer) Asbestos workers (lung cancer) Radium dial painters (oral

cancer)


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Epidemiological Measures

Frequency measures: incidence; prevalence

Effect measures: risk ratio (relative risk); odds ratio

Impact measures: attributable fraction; prevented fraction

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2x2 Table

Disease No Disease

Exposed a = Exposed Cases

b = Exposed Non-Cases

Total exposed (a + b)

Not Exposed c = Non-Exposed Cases

d = Non-Exposed Non-Cases

Total non-exposed

(c + d)

Total Cases Total Non-Cases

n =

a + b + c + d

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INCIDENCE RATES INCIDENCE AMONG THE EXPOSED (NEW CASES AMONG THE SMOKERS) = (A/ A+B) INCIDENCE AMONG THE NON-EXPOSED (NEW CASES AMONG THE NON-SMOKERS) =

(C/ C+D )


Risk Calculations

Relative Risk or Risk Ratio (RR) (RR) = [A / (A+B)] / [C / C +D)] (incidence

in the exposed (smokers) / incidence in the non-exposed (non-smokers)

Attributable risk (AR = excess risk/ incidence among the exposed (AETIOLOGICAL FRACTION)

Population attributable risk (PAR) = incidence in the total population minus incidence among the non-exposed.


RR Interpretations Unity: exposure has no effect on outcome

in the studied population (cohort) More than 1: exposed have a higher risk of

developing the outcome, compared to the unexposed

Less than 1: either no relationship, or a “protective” one exists (e.g. effect of interventions, immunization, health education, management, etc)


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Frequency & Effect Measures

Design Frequency Effect Calculation

Cross-sectional

Prevalence Odds Ratio ad / bc

Case-control None Odds Ratio ad / bc

Cohort Incidence Risk Ratio Ie [a / a + b] /

Iue [c /c + d]

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References

1. Last JM. A dictionary of epidemiology. 5th edition. New York, Oxford, Toronto: Oxford University Press, 2008.

2. Gordis L. Epidemiology. Fourth edition. Philadelphia, London, New York, Sydney: W.B. Saunders, 2009.

3. Beaglehole R, Bonita R, Kjellstrom T. 2nd edition. Basic epidemiology. Geneva: WHO, 2006.

4. Friis RH, Sellers TA. Epidemiology for public health practice. Gaithersburg, MD: Aspen Publishers, 2009.

5. Kelsey JL, Thompson WD, Evans AL. Methods in observational epidemiology. New York: Oxford University Press, 1986.


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Cohort Study Designs