Several models exist for the observed numberZof correct answers in a multiple choice test. One of these is the binomial error model which assumes that the conditional distribution ofZgiven the proportion‐correct true score is binomial. The specification of a beta distribution to the proportion‐correct true score has led to the development of the beta‐binomial model. Recently, extensions of the beta‐binomial model have been given that allow for guessing. LettingXbe the number of known items in the test the regression ofXonZhas received much attention. In the present paper it is argued that the numberWof known items in the item pool is of interest rather thanX, and the regression ofWonZis