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Study types Cohort and casecontrol studies

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Clinical trials. Case-control study. Study Population. Cases. Controls. Exposed ... All patients in a hospital department at a given time. Selection of Controls ... – PowerPoint PPT presentation

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Title: Study types Cohort and casecontrol studies


1
Study typesCohort and case-control studies
  • Charlotte Glümer
  • Bendix Carstensen
  • STAR-course
  • Epidemiology and Biostatistics
  • Aurangabad 2005

2
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

3
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

4
Case-control study
Exposed
Cases
Non-exposed
Study Population
Exposed
Controls
Non-exposed
5
Cohort study / Follow-up study
6
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.

7
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

8
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

9
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

10
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)

11
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

12
2 minutes
13
Cohort studies
  • Retrospective
  • Exposure Disease
  • Yes ?
  • No ?
  • Prospective
  • Exposure Disease
  • Yes ?
  • No ?
  • Ambidirectional

14
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

15
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

16
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

17
Data Collection
18
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

19
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

20
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
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