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Biostatistics coursePart 2
Types of studies in epidemiology
Dr. en C. Nicolas Padilla Raygoza
Departrment of Nursing and Obstetrics
Division of Health Sciences and Engineering
University of Guanajuato Mexico
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.
Professor Titular A, Full time, University of Guanajuato. [email protected] [email protected]
Competencies
The reader will differentiate between observational and experimental studies.
The reader will know advantages and disadvantages of both types of studies.
Types of studies
Types of epidemiological studies
ObservationalExperimental
Quasi-Experi-mental
Randomized Controlled
studies
Cases and controls
Cases or report of cases Cohort Cross sectional Ecologic
Cases in series
Advantages: They are easy to write. The observations are useful to other
researchers. Disadvantages:
There are a lot of bias.
Cases and controls studies
Exposed
Non-exposed
Exposed
Non-exposed
TimeBeginning of studyDirection of research
Cases
Controls
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.
Nested cases and controls studies
Cohort selected to study
Non-exposed
Exposed
With outcome
Without outcome
Cases
Sample of controls
With outcome
Without outcome
Cases
Sample of controls
Beginning of study
Time
Cohort studies
Selection of a cohort for study
Non-exposed
Exposed With outcome
Without outcome
With outcome
Without outcome
Beginning of study
Time
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.
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.
Cross sectional studies
Subjects selected to study
Beginning of study
Exposed with outcome
Exposed without outcome
Non-exposed with outcome
Non-exposed without outcome
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
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.
Experimental studies
Classification Randomized clinical trials. Quasi experimental. With historic controls.
Experimental studies Outcome
Subjects that participate
Outcome
Controls
Beginning of study Intervention Time
Without outcome
Without outcome
Exposed
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.
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.
Experimental studies
Cross-design There are clinical trials with auto controls. The same group work as control group.
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.
Experimental studies
Experimental group
Subjects that participate
Without outcome
Controls
Without outcome
Without outcome
Outcome
Outcome
Experimental group
Outcome
Without outcome
Outcome
Controls
Beginning of study
Intervention Intervention Time
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.
Experimental studies
Subjects
Without outcome
With outcome
Results of a previous study
Without outcome
Beginning Intervention only in subjects Timeof study
With outcome
Experimental studies
Advantages: Give strong evidence 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.
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.
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.
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.htm) (Accesed October 2008).
3.- Hennekens CH, Buring J, Mayrent SL. Epidemiology in Medicine. Boston, Little Brown and Company, 1987.
