Textbook calculations of statistical power or sample size follow from formulas that assume that the variables under consideration are measured without error. However, in the real world of behavioral research, errors of measurement cannot be neglected. The determination of sample size is discussed, and an example illustrates blocking strategy.…

Derivation of asymptotic variance-covariance matrices for item and personal parameters in item response models is demonstrated for one- and two-parameter models using maximum likelihood estimation. The results can be used in incomplete designs and in estimation of the accuracy of various designs beforehand. (TJH)