Bias is systematic error in data collected to address a research question. In contrast to random errors, which are randomly distributed and therefore even out across people or groups studied, biases are errors that are systematically related to people, groups, treatments, or experimental conditions and therefore cause the researcher to overestimate or underestimate the measurement of a behavior or trait. Bias is problematic because it can endanger the ability of researchers to draw valid conclusions about whether one variable causes a second variable (threats to internal validity) or whether the results generalize to other people (threats to external validity). Bias comes in many forms, including sampling bias, selection bias, experimenter expectancy effects, and response bias.

Sampling Bias

Human participants in studies generally represent a subset of the ...

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