In item response theory (IRT), the guessing parameter is a term informally used for the lower asymptote parameter in a three-parameter-logistic (3PL) model. Among examinees who demonstrate very low levels of the trait or ability measured by the test, the value of the guessing parameter is the expected proportion that will answer the item correctly or endorse the item in the scored direction. This can be understood more easily by examining the 3PL model:

where θ is the value of the trait or ability; P(θ) is the probability of correct response or item endorsement, conditional on θ; ai is the slope or discrimination for item i;bi is the difficulty or threshold for item i; and ci is the lower asymptote or guessing parameter for item i. ...

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