Lecture 21: Non-experimental intervention studies (cont) - PowerPoint PPT Presentation

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Lecture 21: Non-experimental intervention studies (cont)

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Lecture 21: Non-experimental intervention studies (cont) Describe the following sources of bias in the evaluation of screening programs: - Length-biased sampling ... – PowerPoint PPT presentation

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Title: Lecture 21: Non-experimental intervention studies (cont)


1
Lecture 21 Non-experimental intervention
studies (cont)
  • Describe the following sources of bias in the
    evaluation of screening programs
  • - Length-biased sampling
  • - Lead time bias
  • Compare experimental vs non-experimental
    intervention study designs, taking into account
    the following considerations
  • - Ethical issues
  • - Feasibility and practicality
  • - Validity (internal and external), precision,
    and efficiency

2
Evaluation of screeningExample evaluation of
the effectiveness of breast cancer screening (HIP
study)
  • 1st RCT of breast cancer screening
  • Study population Members of HMO
  • Intervention Invitation to receive annual
    mammography and clinical exam (3 years)
  • Possible outcomes
  • survival rate (1 year, 5 year)
  • case-fatality rate
  • mortality rate
  • Which would you use?

3
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4
Bias in RCTs of screening
  • Definition of time zero?
  • Date of first symptoms?
  • Date of detection?
  • Date of diagnosis?
  • Bias if difference in time zerobetween study
    groups
  • screening/early detection intervention shifts
    time zero
  • intervention appears to lengthen time to outcome
    without real change in prognosis
  • lead time bias
  • length bias

5
Other types of bias in studies of interventions
  • Hawthorne effect
  • Non-specific effect of being in a study
  • Prevention?
  • Contamination bias
  • Control group receives some component(s) of
    intervention
  • Prevention?
  • Confounding variables
  • Variables associated with intervention group and
    outcome, not in causal chain
  • Prevention?

6
Internal vs external validity
  • Internal validity
  • Lack of bias in study
  • External validity
  • Generalizability
  • Representativeness of study sample

7
Selection of study design for evaluation of
interventions
  • Observational study design
  • Cohort
  • Case-control
  • Quasi-experimental study design
  • Non-equivalent control group
  • Time-series single or multiple (controlled)
  • Experimental
  • RCT

8
Considerations in selection of a study design
  • Cost
  • Feasibility
  • Ethical issues
  • Internal validity
  • External validity
  • Credibility
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