potential errors in epidemiologic studies l.
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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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Presentation Transcript
learning objectives
Learning Objectives
  • Understand the concept of bias
  • Recognize the methods to prevent bias
  • Know the methods to evaluate the impact of bias
performance objectives
Performance Objectives
  • Prevent bias
  • Evaluate bias
  • Improve validity



Lack of





selection bias
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
Sources of Selection Bias
  • Design
  • Sampling
  • Autopsy series
  • Ascertainment
selection bias cont
Selection Bias(cont.)
  • Berkson
  • Self-selection (Response)
  • Healthy worker effect
  • Non-response
information bias observation bias measurement bias
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
Sources of Information Bias
  • Recall
  • Prevarication
  • Reporting
  • Loss of follow-up (withdrawal)
  • Missing data
sources of information bias cont
Sources of Information Bias (cont.)
  • Digit preference
  • Observer (interviewer)
  • Instrumental
sources of information bias cont12
Sources of Information Bias (cont.)
  • Detection
  • Work-up
  • Lead time
  • Length
information bias
Information Bias




control of bias
Control of Bias





Prevention of Bias


Sample Size

Study design

Sources of data collection

Methods of data collection

Content of information


Probability Sampling

  • Simple random
  • Systematic
  • Stratified random
  • Cluster
sample size
Sample Size

Missing Information

Increase Sample Size

study design
Study Design
  • Appropriate study design
  • Comparable study groups
  • Randomization
  • Blind study
source of data collection
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
Methods of Data Collection
  • Standard tools for data collection
  • Standard administration of tools
content of information
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
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

Evaluation of the role of bias




interpretation of results
Interpretation of results
  • Identification of inevitable bias
  • Control for missing information


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



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.


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.