IRT distinguishes the item parameters from the person parameters, which is a very convenient property. The item parameters can be estimated separately during item pre-testing. The calibrated items can be stored in an item bank, and during test administration some of them can be selected adaptively to estimate the person parameters. Logistic IRT models are most commonly applied for scoring dichotomous items. The 3-parameter logistic model (3PLM) can be formulated as: