Bias

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Deviation of results from the true values or process leading to such deviation.

Selection (Sampling) Bias - Spectrum Bias: The sample population chosen is not representative of the population at risk. An appropriate spectrum of patients were not included in the study (e.g. patients with advanced disease were compared with healthy nondiseased patients.

Measurement Bias: Being studied can affect outcome. Also how you measure can affect outcome.

Confounding Bias: Occurs when two factors are closely associated and the effects of one confuses or distorts the effects of the other factor on the outcome. The distorting factor is a confounding variable. Knowledge (factor) is measured by scores on standardized exams. An unmeasured factor (confounding variable) is test taking ability on standardized exams.

Recall Bias: The recall of exposures or events may differ in cases and controls. Questions may be asked more times and more intensively in cases compared to controls. Patients with the disease are more likely to carefully consider whether or not an exposure occurred.

Referral Bias(Centripetal Bias): Physicians and medical centers may attract individuals with specific disorders or exposures.

Volunteer Bias: Volunteers may exhibit exposures or outcomes which may differ from nonvolunteers (e.g. volunteers tend to be healthier).

Withdrawal Bias: Patients who withdraw from studies may differ systematically from those who remain.

Attention Bias: (Hawthorne effect): When subjects systematically alter their behavior when they are being observed.

Therapeutic Personality Bias: Occurs when the observer is not blinded. The observer's beliefs about therapeutic effectiveness may influence outcomes and their measurements.

Investigator Bias: Occurs when the interviewer is aware (not blinded) of the outcome variable. An unblinded interviewer may be more vigorous in searching for the exposure of interest.

Gold Standard Review Bias: A form of investigator bias that occurs when the investigator knows the results of the gold standard test when the new diagnostic test in interpreted.

Index Test Review Bias: A form of investigator bias that occurs when the investigator knows the results of the new diagnostic test when the gold standard test in interpreted.

Verification Bias: Occurs when patients with negative test results are not evaluated with the gold standard test.