1. Epidemiologic study designs
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Epidemiologic study designs
Dr. Sunita Dodani
Assistant Professor
Family Medicine, CHS
THE AGA KHAN
UNIVERSITY
PAKISTAN

2. Learning Objectives
To understand the concepts of different study designs
To learn about the advantages and disadvantages of several study designs

3. Performance Objectives
After this lecture the student will be able
To recognize concepts of different study designs
To appropriately use a study design in research projects

4. Epidemiologic study designs
The basis for the lecture is the distinction between descriptive epidemiology and analytic epidemiology
Descriptive epidemiology:
seeks to measure the frequency in which diseases occur or collect descriptive data on possible causal factors.
Analytic epidemiology:
attempts to specify in more detail the causes of a particular disease”

5. Epidemiologic study designs
Types of Epidemiologic Observational Studies
based on:
type of sampling from population - based on Exposure &/or Disease or neither
temporal sequence of observation - one time point, forward, backwards

6. Epidemiologic study designs
Types of Studies
A. Experimental - study factor is manipulated by the investigator
Types
1. Pure versus Quasi-experimental
2. Laboratory versus real world
B. Observational - no manipulation of study factor by the investigator
1. Descriptive versus Analytic
2. Retrospective versus Prospective

7. Epidemiologic study designs
Factors Important in Study Design
A. Specific, testable hypotheses - NOT a fishing expedition
B. Biases
1. Internal validity
2. External validity

8. Epidemiologic study designs
Case series
Case Series report new diseases or health related problems.
They may provide some descriptive data on exposures to potential causal factors

9. Epidemiologic study designs
Cross-sectional studies
Cross-Sectional Studies measure existing disease and current exposure levels.
They provide some indication of the relationship between the disease and exposure or non-exposure

10. Epidemiologic study designs
Cross Sectional Studies (contd)
sample without knowledge of Exposure or Disease
sample at one point in time
Mostly prevalence studies/surveys

11. Epidemiologic study designs
Cross Sectional Studies (contd)
Advantages
Good design for hypothesis generation
Can estimate overall and specific disease prevalence and sometimes rates
Can estimate exposure proportions in the population
Can study multiple exposures or multiple outcomes or diseases

12. Epidemiologic study designs
Cross Sectional Studies (contd)
Advantages
Relatively easy, quick and inexpensive
No issue of subjecting any animals or producers to particular treatments
Best suited to studying permanent factors (breed, sex, blood-type)
Often good first step for new study issue

13. Epidemiologic study designs
Cross Sectional Studies
Disadvantages
Impractical for rare diseases
Not a useful type of study for establishing causal relationships
Confounding is difficult to control
No control over sample size for each exposure by disease subclass

14. Epidemiologic study designs
Cross Sectional Studies
Disadvantages
Problems with temporal sequence of data
hard to decide when disease was actually acquired
disease may cure the exposure
miss diseases still in latent period
recall of previous exposure may be faulty

15. Epidemiologic study designs
Case-control studies
Case-Control Studies identify existing disease/s and look back in previous years to identify previous exposures to causal factors.
Cases are those who have a disease.
Controls are those without a disease.
Analyses examine if exposure levels are different between the groups.

16. Epidemiologic study designs
Case-control studies
Advantages
Best design for rare diseases
Can be accomplished quickly since events of interest have already occurred
Can study several potential exposures at the same time
Lends itself well to hospital-based studies and outbreaks

17. Epidemiologic study designs
Case-control studies
Disadvantages
Problems with temporal sequence of data
Hard to decide when disease was actually acquired
Disease may cure the exposure
Miss diseases still in latent period
Can’t calculate incidence, population relative risk or attributable risk
HIGH potential for bias

18. Epidemiologic study designs
Cohort Studies
In a cohort study, subjects with an exposure to a causal factor are identified and the incidence of a disease over time is compared with that of controls (persons who do not have the exposure).
In a longitudinal study, subjects are followed over time with continuous or repeated monitoring of risk factors or health outcomes, or both.

19. Epidemiologic study designs
Cohort Studies
Advantages
Can get best assessment of exposure and can deal with changes in exposure
May be only design if exposure needs to be measured directly
Allows study of rare exposures
Only way to get prospective information for rapidly fatal diseases
Good for establishing temporal sequence and natural history of disease
Can examine multiple outcomes linked to exposure - often find other effects than

20. Epidemiologic study designs
Cohort Studies
Advantages
Can estimate overall and specific disease rates, usually incidence
Researcher selects, measures & records data
Lower potential for bias than a case-control study - no recall and selection bias
Results are considered more conclusive than results from case-control studies
The longer a cohort study continues, the stronger it can become

21. Epidemiologic study designs
Cohort studies
Disadvantages
Impractical for rare diseases - even if risk is high, no cases may occur or results
may not be statistically significant
Larger sample size than case-control
Data collection is usually very expensive
Long time commitment for follow-up

22. Epidemiologic study designs
Other designs:
Time series - test if incidence of disease changes in a population over time
Hybrid - often what is seen in practice
Can be efficient and match necessity
Can lead to bias and disaster
Meta-analysis
Combining results from a range of published studies
Established methodology, not just literature review

23. Epidemiologic study designs
What type of study to chose depends on:
what is the research question/ objective
Time available for study
Resources available for the study
Common/rare disease or production problem
Type of outcome of interest
Quality of data from various sources
Often there are multiple approaches which will all work
Choosing an established design gives you a huge head start in design, analysis and eliminating biases