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Understanding lack of validity: Bias. Objectives. To define and discuss the concept of bias To define and discuss selection bias and information bias. To discuss exposure and outcome identification bias To discuss the results of information bias

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Presentation Transcript
  • To define and discuss the concept of bias
  • To define and discuss selection bias and information bias.
  • To discuss exposure and outcome identification bias
  • To discuss the results of information bias
  • Combine selection/information bias: detection bias, incidence-prevalence bias, temporal bias
bias definition
Bias - Definition
  • Is defined as the result of a systematic error in the design or conduct of a study
  • Results from:
    • Flaws in the method of selection of study participants
    • Procedures for gathering relevant exposure and/or disease information
bias definition5
Bias - definition
  • Bias exists when, on the average, the results of an infinite number of studies differs from the true results
  • Most biases related to study design and procedures can be classified into:
    • Selection bias
    • Information bias
information bias
Information bias
  • A systematic tendency for individuals selected for inclusion in the study to be erroneously placed in different exposure/outcome categories (misclassification)
  • Examples: recall bias – ability to recall past exposure is dependent on case-control status.
selection bias
Selection bias
  • Systematic error in the ascertainment of study subjects
  • Berksonian bias – when this bias occurs in case-control studies of hospitalized patients.
recall bias
Recall bias
  • When recall of past exposure error differs between cases and controls
  • Methods to prevent recall bias
    • Review of other documentation (eg. pharmacy or hospital charts)
    • Using proxy respondents (spouse, parent, etc)
    • Using biological markers
interviewer bias
Interviewer bias
  • When the disease status is not masked and the interviewer differentially ascertains exposure status
  • Methods to reduce this error: sub-study interviews, masking of case-control status, standardization in how you ask the question
outcome identification bias
Outcome identification bias
  • May occur in both case-control and cohort
  • Usually due to an imperfect definition of the outcome or errors in the data collection stage.
    • Observer bias
    • Respondent bias
results of information bias
Results of information bias
  • Non-differential misclassificaton (systematic)
  • Differential (non-systematic)
effect of misclassification of a confounding variable
Effect of misclassification of a confounding variable
  • Results in an imperfect adjustment due to residual confounding
  • Can lead to spurious conclusions
medical surveillance bias
Medical surveillance bias
  • Can be selection or information bias
  • This bias is most likely to occur when the exposure is a medical condition or therapy that leads to frequent/detailed checkups (eg:diabetes, OCP)
cross sectional bias
Cross-sectional bias
  • Incidence-prevalence bias – results from the inclusion of prevalent cases into the study (important when duration of disease is differential)
  • Temporal bias – don’t know which came first exposure or disease
lead time bias
Lead time bias
  • The time by which a diagnosis can be advanced by screening
  • Occurs when estimating survival time
publication bias
Publication bias
  • Assumption that published papers should be unbiased and represent an unbiased sample of the theoretical “population” of unbiased studies
  • Papers with statistically significant results are more likely to be published