Abstract
Occasionally mixtures of two binomials with one known success parameter are met. An example in educational testing is the mastery or random guessing model, in which an examinee is supposed either to master the items or not to master them and to guess blindly. This paper gives moment estimators for such mixtures and presents results from a Monte Carlo investigation into their statistical properties. The results suggest excellent estimators which can safely be used in most instances. It is also indicated how the properties of these estimators relate to those of moment estimators in case both success parameters are unknown. Finally, it is pointed out that in situations in which errors in specifying the true value of the known parameter may occur, it might be prudent to consider this parameter as unknown and to estimate accordingly.
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