1. Biostatistics course Part 2 Types of studies in epidemiology
Dr. en C. Nicolás Padilla Raygoza
Facultad de Enfermería y Obstetricia de Celaya
Universidad de Guanajuato México

2. Biosketch Medical Doctor by University Autonomous of Guadalajara.
Pediatrician by the Mexican Council of Certification on Pediatrics.
Postgraduate Diploma on Epidemiology, London School of Hygine and Tropical Medicine, University of London.
Master Sciences with aim in Epidemiology, Atlantic International University.
Doctorate Sciences with aim in Epidemiology, Atlantic International University.
Associated Professor B, School of Nursing and Obstetrics of Celaya, university of Guanajuato.

3. Competencies
The reader will differentiate between observational and experimental studies.
The reader will know advantages and disadvantages of both types of studies.

4. Types of studies
Observational studies. We observe one or more groups of subjects and their characteristics are described and/or analyzed.
Experimental. We can manipulate variables.
Series of cases. To describe characteristics of a patients group. Leads to the generation of hypothesis; use a short time period and there is not a control group.
Cases and controls. The beginning is with the presence or absence of outcome y review before in the time to detect risk factors.
Cohort. The beginning is with a subjects group with outcome; then, we classified them in exposed or non-exposed, and follow in the time to see if they develop the outcome.
Cross-sectional. We measure the exposure and the outcome in a subjects group simultaneously, in a point of the time.
Experimental. The beginning is with a cohort and randomize it to receive the intervention (experimental group) or not receive it (control group); then we follow to the subjects in the time to see if develop the outcome of interest.
Ecologic. The main characteristic is that study groups not individuals.

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

6. Cases and controls studies
Exposed
Cases
Non-exposed
Exposed
Controls
They are called retrospective because of their direction in the time. Sometimes are matched cases with controls to make them more similar.
Non-exposed
Direction of research
Time
Beginning of study

7. 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.

8. Nested cases and controls studies
With outcome
Cases
Exposed
Without outcome
Cohort selected to study
Sample of controls
With outcome
Cases
It begin as cohort study, classify the subjects as exposed or non-exposed; in the follow up, some participants develop the outcome, and they are the cases and a sample of subjects without outcome are controls and with them analyzed the cases-controls study. An important advantage is that cases and controls are members of the same population, because the probability of selection bias is minimized.
Another advantage is that the information about the exposure is collected before of the develop of outcome, and the probability of information bias is minimized.
Without outcome
Non-exposed
Sample of controls
Beginning of study
Time

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

10. 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.
A cohort is a subjects group or things that they have some in common and do not have the outcome of interest.

11. Cohort studies Advantages: Disadvantages:
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.

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

13. 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
They are called prevalence studies. They help us to know the economic, social burden of a disease in a population in a point of time.
They can be: descriptive or analytic. The descriptive only show the frequency of exposure and of outcome in population in a point of time; in analytic studies we search to show the exposure and outcome in a point of time, searching associations with Prevalence Ratio or Odds Ratio.

14. 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.

15. Experimental studies Classification Randomized clinical trials.
Quasi experimental.
With historic controls.
También se les llama pruebas clínica o estudios intervencionales.

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

17. 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.

18. 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.

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

20. 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.

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

22. 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.

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

24. Experimental studies Advantages: Disadvantages:
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.

25. 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.

26. Ecologic studies Advantages: Disadvantages: 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.

27. Bibliography
1.- Gordis L. Epidemiology. Phialdelphia, W.B. Saunders Company, 1996.
2.- Songer T. Study designs in epidemiologic research. Supercourse, 2005 ( (Accesed October 2008).
3.- Hennekens CH, Buring J, Mayrent SL. Epidemiology in Medicine. Boston, Little Brown and Company, 1987.