1. Types of studies in
epidemiology

2. Types of studies
Types of epidemiological
studies
Experimental
Observational
Randomized Quasi-
Controlled Experi-
studies mental
Cases or report Cases and Cohort Cross sectional Ecologic
of cases controls

3. Cases in series
 Advantages:
 They are easy to write.
 The observations are useful to other
researchers.
 Disadvantages:
 There is a lot of bias.

4. Cases and controls studies
Exposed Cases
Non-exposed
Exposed Controls
Non-exposed
Direction of research Beginning of study Time

5. Cases and controls studies
 Advantages:
 They are adequate to study rare outcomes.
 They are adequate to outcomes with long latency
period.
 They are cheap and easy to apply.
 It is not necessary to wait to present outcome.
 Disadvantages:
 A lot of bias.
 They depend on the quality of registries.
 Control group should be adequately selected,
because they represent the population without the
outcome.

6. Cohort studies
With outcome
Cases
Exposed
Without
outcome
Cohort selected Sample of
to study controls
With outcome
Cases
Without
Non- outcome Sample of
exposed controls
Beginning of Time
study

7. Cohort studies
Exposed With outcome
Selection of a Without
cohort for study outcome
With outcome
Without
Non-exposed outcome
Beginning of Time
study

8. Cohort studies
 Subjects are selected because do not have
the outcome and they are classified if have or
not have the risk factor (exposure).
 We follow up to prove if they develop the
outcome.
 The cohort study can be prospective if the
follow up is forward in the time or it can be
retrospective (historic), if it go back in the
time.

9. Cohort studies
 Advantages:
 They are adequate to know the causes of an
outcome.
 To know the natural history of disease.
 They adequate when the exposure is rare.
 They are useful when we study two or more
outcome at the same time.
 Disadvantages:
 They take a long time.
 They are expensive.
 Subjects can be lost in the follow up.
 They are not adequate for study rare
outcomes.

10. Cross sectional studies
Exposed with outcome
Subjects
selected to
study
Exposed without
outcome
Non-exposed with
outcome
Non-exposed without
outcome
Beginning of study

11. Cross sectional studies
 Analyze data of a subjects group in a point of
time.
 Describe a disease and its importance for the
population.
 Define the needed on health.
 They can be classified in:
 Descriptive
 Analytic

12. Cross-sectional studies
 Advantages:
 They are useful to know the burden of a
disease in a group.
 Useful to evaluate diagnostic procedures.
 To study common risk factors.
 To study common outcomes.
 Disadvantages:
 Populations little willing to collaborate.
 The sample can not be representative from
the population.
 It is not useful to search causes of the
outcome.

13. Experimental studies
 Classification
 Randomized clinical trials.
 Quasi experimental.
 With historic controls.

14. Experimental studies
Exposed
Outcome
Without
Subjects that outcome
participate
Outcome
Controls Without
outcome
Beginning of study Intervention Time

15. Experimental studies
 They are called clinical trials.
 It is administrated an intervention to a group,
randomize selected and we do not know what
is receiving (blind). The group that does not
receive the intervention, it is a control group.
 The allocation of subjects in experimental or
control group is given by chance.
 By ethics reasons, only it is permitted
beneficial interventions.

16. Experimental studies
 Blind single is when the subjects do not know
what intervention are receiving.
 Double blind is when neither subjects nor
researcher know what intervention are
receiving each subject.

17. Experimental studies
 There are clinical trials with auto controls.
 The same group work as control group.

18. Experimental studies
 There are cross design where it is
administrated an intervention (1) to
experimental group and another (2) in a
control group.
 After, interventions are suspended, and left a
space (wash out period) without it, then the
intervention 1 is administrated to control
group and intervention 2 is administrated to
experimental group.

19. Experimental studies
Outcome
Outcome
Experimental
group Controls
Subjects that
participate
Without
Without outcome
outcome
Outcome Outcome
Experimental
Controls Without group
outcome
Without
outcome
Beginning of Intervention Intervention Time
study

20. Experimental studies
 There are clinical trials with external controls.
 We compare the results with the results of
another researcher or with the results of a
previous study.
 Also, they are called historic controls.

21. Experimental studies
With outcome
Subjects
Without
outcome
With outcome
Results of a previous
study
Without outcome
Beginning Intervention only in subjects Time
of study

22. Experimental studies
 Advantages:
 Give evidence strong of causality.
 There are less bias.
 Historic controls are used in preliminary
studies.
 Disadvantages:
 Inappropriate use of historic controls lead a
severe mistakes.
 Expensive.
 They need time.

23. Ecologic studies
 Compare exposure and the outcome between
groups.
 Measure the exposure and outcome, in the
group as all.
 They are only studies that offer to study
differences between groups.

24. Ecologic studies
 Advantages:
 Fast
 Cheap
 Use routinely data
 Disadvantages:
 They did not take into account to the
individual.
 They depend on the quality of routinely data
 They are difficult to interpret.

25. Bibliography
 1.- Gordis L. Epidemiology. Phialdelphia,
W.B. Saunders Company, 1996.
 2.- Songer T. Study designs in epidemiologic
research. Supercourse, 2005 (
http://www.pitt.edu/~super1/lecture/lec19101/index
) (Accesed October 2008).
 3.- Hennekens CH, Buring J, Mayrent SL.
Epidemiology in Medicine. Boston, Little
Brown and Company, 1987.