Biostatistics course Part 2 Types of studies in epidemiology Dr. en C. Nicolas Padilla Raygoza...

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. padillawarm@gmail.com raygosan@ugto.mx

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.