A question of perspective

1
A question of perspective

• Types of epidemiological studies
• FETP India

2
Competency to be gained from this lecture

• Choose a design adapted to the objective of a
  study

3
Key areas

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

4
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?)

5
Basic relation between exposure, time and outcome
Referent exposure period
Time
Outcomes(e.g., Disease)
Exposure
6
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

7
Key areas

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

Cohorts
8
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
9
Potential objectives of a cohort study

• Descriptive
• Estimate incidence
• Analytic
• Compare the incidence of a disease in various
  subgroups
• Exposed
• Unexposed

Cohorts
10
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
11
Presentation of the data of an analytical cohort
study in a 2 x 2 table
Ill Non-ill Total Exposed a b ab Non-exposed c
d cd Total ac bd abcd
Cohorts
12
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
13
Key areas

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

Case control studies
14
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
15
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
16
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
17
Presentation of the data of a case control study
in a 2 x 2 table
Cases Controls Total Exposed a b - Non-exposed c
d - Total ac bd -
Case control studies
18
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
19
Key areas

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

Cross sectional studies
20
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
21
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
22
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
23
Presentation of the data of an analytical
cross-sectional study in a 2 x 2 table
Ill Non-ill Total Exposed a b ab Non-exposed c
d cd Total ac bd abcd
Cross sectional studies
24
Limitations of causal inference in analytical
cross-sectional studies

• Prevalent cases
• Exposure and outcome examined at the same time

Cross-sectional studies
25
Examples of cross-sectional studies

• Census
• Population surveys

Cross sectional studies
26
Compared logic and logistics of cohorts, case
control and cross-sectional studies
Cohorts Case-control studies Cross-sectional studies
Logic Prospective Retrospective Cross-sectional
Logistic Prospective Retrospective Prospective Retrospective Cross-sectional
27
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
28
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
29
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
30
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