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Potential Errors In Epidemiologic Studies

Potential Errors In Epidemiologic Studies. Bias . III. Dr. Sherine Shawky. Learning Objectives. Understand the concept of bias Recognize the methods to prevent bias Know the methods to evaluate the impact of bias. Performance Objectives. Prevent bias Evaluate bias Improve validity.

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Potential Errors In Epidemiologic Studies

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  1. Potential Errors In Epidemiologic Studies Bias III. Dr. Sherine Shawky

  2. Learning Objectives • Understand the concept of bias • Recognize the methods to prevent bias • Know the methods to evaluate the impact of bias

  3. Performance Objectives • Prevent bias • Evaluate bias • Improve validity

  4. Inaccuracy Bias Lack of Validity

  5. Bias Selection Information

  6. Selection Bias Error due to systematic difference between the characteristics of the people selected for a study and those who are not.

  7. Sources of Selection Bias • Design • Sampling • Autopsy series • Ascertainment

  8. Selection Bias(cont.) • Berkson • Self-selection (Response) • Healthy worker effect • Non-response

  9. Information Bias(Observation Bias, Measurement Bias) Error due to systematic differences in the way data on exposure or outcome are obtained from various groups leading to misclassification of study subjects

  10. Sources of Information Bias • Recall • Prevarication • Reporting • Loss of follow-up (withdrawal) • Missing data

  11. Sources of Information Bias (cont.) • Digit preference • Observer (interviewer) • Instrumental

  12. Sources of Information Bias (cont.) • Detection • Work-up • Lead time • Length

  13. Information Bias Misclassification Non-random Random

  14. Control of Bias Prevent Study Evaluate

  15. Prevention of Bias Sampling Sample Size Study design Sources of data collection Methods of data collection Content of information

  16. Sampling Probability Sampling • Simple random • Systematic • Stratified random • Cluster

  17. Sample Size Missing Information Increase Sample Size

  18. Study Design • Appropriate study design • Comparable study groups • Randomization • Blind study

  19. Source of Data Collection • Well defined population • Standard source of information • Multiple standard sources to confirm information • Methods to assure participation and compliance

  20. Methods of Data Collection • Standard tools for data collection • Standard administration of tools

  21. Content of Information • Standard definition for exposure and outcome • Multiple questions seeking same information • Information on several items related to the same observation

  22. Content of Information (cont.) • Standardize the time for completeness of study tools • Scoring of comprehension and reliability of used tool by study personnel

  23. Evaluation of the role of bias Repeatability Results Validity

  24. Interpretation of results • Identification of inevitable bias • Control for missing information

  25. Validity When a survey is done and dichotomizes subjects according to exposure and outcome, validity of results can be analyzed by comparing the survey results to standard reference test in contingency table

  26. Survey test vs. reference test

  27. Repeatability Repeatability could be measured within observers (same observer on same subjects on different occasions) or between observers (different observers on same subjects) and results expressed in contingency table.

  28. Observer 1 vs. Observer 2

  29. Conclusion Identification of possible bias is a difficult exercise but is crucial to improve validity. Bias is most effectively dealt with through careful design and meticulous conduct of study. If potential source of bias is introduced, it is usually difficult to correct for its effect analytically.

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