Biostatistics course Part 2 Types of studies in epidemiology

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Biostatistics coursePart 2Types of studies in
epidemiology

• Dr. en C. Nicolás Padilla Raygoza
• Facultad de Enfermería y Obstetricia de Celaya
• Universidad de Guanajuato México

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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.
• padillawarm_at_gmail.com

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Competencies

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

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Types of studies
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Cases in series

• Advantages
• They are easy to write.
• The observations are useful to other researchers.
• Disadvantages
• There is a lot of bias.

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

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Nested cases and controls studies

With outcome
Cases
Exposed
Without outcome
Cohort selected to study
Sample of controls

With outcome
Cases

Without outcome
Non-exposed
Sample of controls
Beginning of study
Time
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Cohort studies

With outcome
Exposed
Selection of a cohort for study
Without outcome

With outcome

Without outcome
Non-exposed
Time
Beginning of study
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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.

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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.

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Cross sectional studies
Exposed with outcome
Subjects selected to study
Exposed without outcome
Non-exposed with outcome
Non-exposed without outcome


Beginning of study
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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

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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.

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Experimental studies

• Classification
• Randomized clinical trials.
• Quasi experimental.
• With historic controls.

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Experimental studies


Outcome
Exposed


Without outcome

Subjects that participate



Outcome


Without outcome
Controls
Beginning of study
Intervention Time
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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.

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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.

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Experimental studies

• There are clinical trials with auto controls.
• The same group work as control group.

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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.

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Experimental studies




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
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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.

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Experimental studies


With outcome
Subjects

Without outcome

With outcome
Results of a previous study

Without outcome
Beginning Intervention only in subjects
Time of study
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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.

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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.

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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.

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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.