The relationship between examinee ability and the accuracy of maximum likelihood item parameter estimation is explored in terms of the expected (Fisher) information. Information functions are used to find the optimum ability levels and maximum contributions to information for estimating item parameters in three commonly used logistic item response models. For the three and two parameter logistic models, examinees who contribute maximally to the estimation of item difficulty contribute little to the estimation of item discrimination. This suggests that in applications that depend heavily upon the veracity of individual item parameter estimates (e.g., adaptive testing or test construction), better item calibration results may be obtained (for fixed sample sizes) from examinee calibration samples in which ability is widely dispersed. (Contains 2 tables, 4 figures, and 11 references.) (Author/SLD)