Nominally and rigorously parallel test forms

When several test forms are used interchangeably, they will not in practice, despite all efforts, be rigorously parallel. A definition of the standard error of measurement appropriate for this type of situation can be provided; however, it will be different from the definition in classical test theory. Appropriate formulas for the standard error of measurement and for other related quantities under both definitions are derived and compared; also sample statistics for estimating these quantities.The writer is indebted to Lee Cronbach and Julian Stanley for many helpful suggestions for revising a draft of this paper. Part of the work was carried out while the writer was Brittingham Visiting Professor at the University of Wisconsin. This work was supported in part by contract Nonr-2752(00) between the Office of Naval Research and Educational Testing Service. Reproduction in whole or in part for any purpose of the United States Government is permitted. The termnominally parallel was suggested by Lee Cronbach.