A question of perspective

Types of epidemiological studies

FETP India

Competency to be gained from this lecture

Choose a design adapted to the objective of a study

Key areas

• Cohort studies• Case control studies• Cross-sectional studies

Observational and interventional studies

• Observational studies Descriptive

(e.g., What is the incidence of measles?) Analytic

(e.g., What are the risk factors for leishmaniasis?)

• Interventional studies Descriptive

(e.g., “demonstration” studies / projects) Analytic

(e.g., What is the effect of this intervention?)

Basic relation between exposure, time and outcome

Exposure

Outcomes(e.g., Disease)

Time

Referent exposure

period

General considerations about analytical studies

• All risk factors studies should be conducted with incident cases

• Prevalent cases expose to difficulties: Selective survival Impossibility to determine when the disease

occurred Impossibility to determine when the

exposure occurred Lack of information about the exposure -

disease sequence prevents causal inference

Key areas

• Cohort studies• Case control studies• Cross-sectional studies

Cohorts

Cohort study

• Recruitment of study participants at risk Population Population sample

• Follow-up over time• Observation for the occurrence of one

or more outcomes

Cohorts

Potential objectives of a cohort study

• Descriptive Estimate incidence

• Analytic Compare the incidence of a disease in

various subgroups• Exposed • Unexposed

Cohorts

Elements defining an analytical cohort study

• Study participants At risk for the outcome Selected on the basis of their exposure

status

• Logic Prospective follow-up to measure incidence

• Logistic Prospective Retrospective

Cohorts

Ill Non-ill Total

Exposed a b a+b

Non-exposed c d c+d

Total a+c b+d a+b+c+d

Presentation of the data of an analytical cohort study in a 2 x 2

table

Cohorts

Known at the beginning of the study

Examples of cohort studies

• Retrospective cohort study to investigate an outbreak Wedding dinner

• Long-term cohort to estimate the incidence of cardio-vascular diseases Framingham

• Clinical trials An example of interventional studies

Cohorts

Key areas

• Cohort studies• Case control studies• Cross-sectional studies

Case control studies

Case control study

• Recruitment of: Case-patients affected with a disease Unaffected control-subjects who are at risk

• Comparison of exposure status• Observation for the presence in the past

of one or more potential risk factors

Case control studies

Objective of a case control study

• Analytic Compare, in terms of exposure status:

• Case-patients affected by a disease • Unaffected control-subjects

Case control studies

Elements defining a case control study

• Study participants Selected on the basis of their disease status

• Logic Retrospective examination of potential

exposures

• Logistic Prospective Retrospective

Case control studies

Cases Controls Total

Exposed a b -

Non-exposed c d -

Total a+c b+d -

Presentation of the data of a case control study in a 2 x 2 table

Case control studies

Fixed at the beginning of the study

Examples of case control studies

• Case control study to investigate an outbreak Risk factor for hepatitis E during an

outbreak in a metropolitan area

• Case control study to investigate the risk factors for a rare disease Creutzfeld-Jacob

Case control studies

Key areas

• Cohort studies• Case control studies• Cross-sectional studies

Cross sectional studies

Cross-sectional study

• Recruitment of study participants Population Population sample

• Observation for the presence of: One or more outcomes One or more exposures

Cross sectional studies

Potential objectives of a cross-sectional study

• Descriptive Estimate prevalence

• Analytic Compare the prevalence of a disease in

various subgroups, exposed and unexposed Compare the prevalence of an exposure in

various subgroups, affected and unaffected

Cross sectional studies

Elements defining an analytical cross-sectional study

• Study participants Selected independently from their disease

or exposure status

• Logic Simultaneous measurement of outcomes

and potential exposures

• Logistic Cross-sectional

Cross sectional studies

Ill Non-ill Total

Exposed a b a+b

Non-exposed c d c+d

Total a+c b+d a+b+c+d

Presentation of the data of an analytical cross-sectional study in a

2 x 2 table

Cross sectional studies

Known simultaneously when the study is completed

Limitations of causal inference in analytical cross-sectional studies

• Prevalent cases• Exposure and outcome examined at the

same time

Cross-sectional studies

Examples of cross-sectional studies

• Census• Population surveys

Cross sectional studies

Compared logic and logistics of cohorts, case control and cross-

sectional studies Cohorts Case-control

studiesCross-sectional studies

Logic •Prospective •Retrospective •Cross-sectional

Logistic •Prospective•Retrospective

•Prospective•Retrospective

•Cross-sectional

Comparing cohorts and case control studies (1)

Case control

Cohort

Rare outcomes Adapted Less adapted

Rare exposures Less adapted

Adapted

Multiple exposures Yes Difficult

Multiple outcomes No Yes

Estimation of incidence No Yes

Comparing cohorts and case control studies (2)

Case control

Cohort

Outcome status Easier Difficult if retrospective

Exposure status Recall bias Easier

Natural history - Can be studied

Use of existing data Yes Yes(Retrospective)

Sample size Smaller Larger

Comparing cohorts and case control studies (3)

Case control

Cohort

Cost Lower Higher

Time needed Short Longer

Follow-up (F/U) N/A Lost to F/U

Logistics Easier More difficult

Concept Difficult Easier

Ethics Easier More difficult

Take-home messages

• Cohort studies go from exposure to outcome(s)

• Case control studies go from outcome to exposure(s)

• Cross-sectional studies examine outcome and exposure at the same time