Study types Cohort and casecontrol studies

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Study typesCohort and case-control studies

• Charlotte Glümer
• Bendix Carstensen
• STAR-course
• Epidemiology and Biostatistics
• Aurangabad 2005

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Aim of epidemiological studies

• To determine distribution of disease
• To examine determinants of a disease
• To judge whether a given exposure causes or
  prevents disease

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Epidemiologic Design Strategies

• Descriptive studies
• Populations
• Correlated studies
• Individuals
• E.g. case-series, case reports, cross-sectional
  surveys
• Analytical studies
• Observational studies
• Case-control studies
• Cohort studies
• Intervention studies
• Clinical trials

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Case-control study
Exposed
Cases
Non-exposed
Study Population
Exposed
Controls
Non-exposed
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Cohort study / Follow-up study
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Selection of cases

• Establish a strict diagnostic criteria for the
  disease
• Examples
• Type 1 diabetes in children severe symptoms,
  very high BG, marked glycosuria, and ketonuria.
• Type 2 diabetes few if any symptoms, Slightly
  elevated BG, diagnosis complicated.

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Selection of cases

• Population-based cases include all subjects or a
  random sample of all subjects with the disease at
  a single point or during a given period of time
  in the defined population
• Danish childhood diabetes register
• Hospital-based cases
• All patients in a hospital department at a given
  time

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Selection of Controls

• Principles of Control Selection
• Study base
• Controls can be used to characterise the
  distribution of exposure
• Comparable-accuracy
• Equal reliability in the information obtained
  from cases and controls ? no systematic
  misclassification
• Overcome confounding
• Elimination of confounding through control
  selection? matching or stratified sampling

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Selection of Controls

• General population controls
• registries, households, telephone sampling
• costly and time consuming
• recall bias
• eventually high non-response rate
• Hospitalised controls
• Patients at the same hospital as the cases
• Easy to identify
• Less recall bias
• Higher response rate

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Ascertainment of Disease and exposure status

• External sources
• Death certificates, disease registries, Hospital
  and physicians records etc.
• Internal sources
• Questionnaires and interviews, information from a
  surrogate (spouses or mother of children),
  biological sampling( e.g. antibody)

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Bias in Case-Control studies

• Selection bias
• Non-response
• Detection bias
• cases and controls are identified not
  independently of the exposure
• Observation bias
• Recall Bias Cases are more likely to remember
  exposure than controls

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2 minutes
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Cohort studies

• Retrospective
• Exposure Disease
• Yes ?
• No ?
• Prospective
• Exposure Disease
• Yes ?
• No ?
• Ambidirectional

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Prospective vs. retrospective Cohort Studies

• Prospective Cohort Studies
• Time consuming, expensive
• More valid information on exposure
• Measurements on potential confounders
• Retrospective Cohort Studies
• Quick, cheap
• Appropriate to examine outcome with long latency
  periods
• Admission to exposure data
• Difficult to obtain information of exposure
• Risk of confounding

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Selection of the Exposed Population

• Sample of the general population
• Geographically area, special age groups, birth
  cohorts (Framingham Study)
• A group that is easy to identify
• Nurses health study
• Special population (often occupational
  epidemiology)
• Rare and special exposure
• Permits the evaluation of rare outcomes

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Selection of the Comparison Population

• Internal Control Group
• Exposed and non-exposed in the same Study
  population (Framingham study, Nurses health
  study)
• Minimise the differences between exposed and
  non-exposed
• External Control Group
• Chosen in another group, another cohort
  (Occupational epidemiology Asbestosis vs. cotton
  workers)
• The General Population

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Data Collection
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Bias

• Selection bias
• Non-response during data collection
• Losses to follow up
• Healthy worker effect
• Misclassification on exposure or event
• Random
• Systematic
• Confounder
• Difference in other risk factors between exposed
  and non-exposed

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Strengths in Cohort vs. Case-control?

• Cohort study
• Rare exposure
• Examine multiple effects of a single exposure
• Minimizes bias in the in exposure determination
• Direct measurements of incidence of the disease

• Case-control study
• Quick, inexpensive
• Well-suited to the evaluation of diseases with
  long latency period
• Rare diseases
• Examine multiple etiologic factors for a single
  disease

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Limitations in Cohort vs. Case-control?

• Cohort study
• Not rare diseases
• Prospective Expensive and time consuming
• Retrospective in adequate records
• Validity can be affected by losses to follow-up

• Case-control study
• Not rare exposure
• Incidence rates cannot be estimated unless the
  study is population based
• Selection Bias and recall bias