The Rasch model is described as a latent trait model which meets the five criteria that characterize reasonable and objective measurements of an individual's ability independent of the test items used. The criteria are: (1) calibration of test items must be independent of particular norming groups; (2) measurement of individuals must be independent of particular items used for measuring; (3) test items must measure a single underlying trait or ability; (4) a more able individual must have a better chance of success with an item than a less able individual; and (5) an individual must have a better chance of success on an easy item than on a difficult item. To illustrate the use of the Rasch model for vertical equating, equal interval scales (EIS) were developed and applied to the Scott, Foresman Comprehensive Assessment Program (CAP), a coordinated series of tests and measures for evaluating students' educational growth. (Author/MH)