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

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

Bias Lack of Validity Inaccuracy

Bias Selection Information

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

Sources of Selection Bias Design Sampling Autopsy series Ascertainment

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

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

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

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

Detection Work-up Lead time Length

Information Bias Misclassification Random Non-random

Control of Bias Prevent Study Evaluate

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

Sampling Simple random Systematic Stratified random Cluster Probability Sampling

Sample Size Missing Information Increase Sample Size

Study Design Appropriate study design Comparable study groups Randomization Blind study

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

Methods of Data Collection Standard tools for data collection Standard administration of tools

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

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

Evaluation of the role of bias Repeatability Results Validity

Interpretation of results Identification of inevitable bias Control for missing information

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

Survey test vs. reference test

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.

Observer 1 vs. Observer 2

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.