In a clinical trial, bias refers to effects that a conclusion that may be incorrect as, for example, when a researcher or patient knows what treatment is being given. To avoid bias, a blinded study may be done.
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1. Systematic discrepancy between a measurement and the true value; may be constant or proportionate and may adversely affect test results. 2. Any trend in the collection, analysis, interpretation, publication, or review of data that can lead to conclusions that differ systematically from the truth; deviation of results or inferences from the truth, or processes leading to deviation. [Fr. biais, obliquity, perh. fr. L. bifax, two-faced] There is no imputation of prejudice, partisanship, or other subjective or emotional factor such as an investigator's desire to achieve a particular outcome. More than 100 varieties of b. have been described but all fall into one of a rather small number of distinct categories: 1. Systematic one-sided variation of measurements from the true value (syn systematic error, instrumental error, or b.). 2. Variation of statistical summary measures (means, rates, measures of association, etc.) from their true values as a result of systematic variation of measurements, other flaws in data collection, or flaws in study design or analysis. 3. Deviation of inferences from the truth as a result of flaws in study design, data collection, or the analysis or interpretation of results. 4. A tendency of procedures in study design, data collection, analysis, interpretation, review or publication, to yield results or conclusions that depart from the truth. 5. Prejudice leading to the conscious or subconscious selection of study procedures that depart from the truth in a particular direction, or to one-sidedness in interpretation of results. This form of b. can arise as a result of shoddy scientific methods, or deliberately when investigators behave fraudulently in order to misrepresent the truth.
- ascertainment b. systematic failure to represent equally all classes of cases or persons supposed to be represented in a sample.
- cross-level b. a b. due to aggregation at the population level of causes and/or effects that are unlike at the individual level; can occur in ecologic studies.
- recall b. systematic error due to differences in accuracy or completeness of recall to memory of past events or experiences.
- reporting b. selective revealing or suppression of information about past medical history, e.g., details of exposure to sexually transmitted diseases.
- response b. systematic error due to differences in characteristics between those who choose or volunteer to take part in a study, and those who do not.
- sampling b. systematic error due to study of a nonrandom sample of a population.

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bi·as (biґəs) 1. (in a measurement process) systematic error. 2. (of a statistical estimator) the difference between the expected value of the estimator and the true parameter value.

Medical dictionary. 2011.

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