Bias (statistics)

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Statistical bias is a feature of a statistical technique or of its results, whereby the expected value of the results differs from the true underlying quantitative parameter being estimated.


A statistic is biased if it is calculated in such a way that it is systematically different from the population parameter of interest. The following lists some types of biases, which can overlap.

  • Selection bias involves individuals being more likely to be selected for study than others, biasing the sample. This can also be termed Berksonian bias.[1]
  • The bias of an estimator is the difference between an estimator's expectations and the true value of the parameter being estimated.
    • Omitted-variable bias is the bias that appears in estimates of parameters in a regression analysis when the assumed specification omits an independent variable that should be in the model.
  • In statistical hypothesis testing, a test is said to be unbiased when the probability of committing a type I error (i.e. false positive) is equal to the significance level.
  • Detection bias occurs when a phenomenon is more likely to be observed for a particular set of study subjects. For instance, the syndemic involving obesity and diabetes may mean doctors are more likely to look for diabetes in obese patients than in thinner patients, leading to an inflation in diabetes among obese patients because of skewed detection efforts.
  • In educational measurement, bias is defined as "Systematic errors in test content, test administration, and/or scoring procedures that can cause some test takers to get either lower or higher scores than their true ability would merit. The source of the bias is irrelevant to the trait the test is intended to measure." [2]
  • Funding bias may lead to selection of outcomes, test samples, or test procedures that favor a study's financial sponsor.
  • Reporting bias involves a skew in the availability of data, such that observations of a certain kind are more likely to be reported.
  • Analytical bias arise due to the way that the results are evaluated.
  • Exclusion bias arise due to the systematic exclusion of certain individuals from the study.
  • Attrition bias arises due to a loss of participants e.g. loss to follow up during a study.[3]
  • Recall bias arises due to differences in the accuracy or completeness of participant recollections of past events. e.g. a patient cannot recall how many cigarettes they smoked last week exactly, leading to over-estimation or under-estimation.
  • Observer bias arises when the researcher subconsciously influences the experiment due to cognitive bias where judgement may alter how an experiment is carried out / how results are recorded.

See also[edit]


  1. ^ Rothman, K.J. et al. (2008) Modern Epidemiology (Lippincott Williams & Wilkins) pp.134-137.
  2. ^ National Council on Measurement in Education
  3. ^ Higgins, Julian PT; Green, Sally (March 2011). Cochrane Handbook for Systematic Reviews of Interventions. The Cochrane Collaboration.