Using Longitudinal Data to Estimate Reliability

Abstract

Werts, Breland, Grandy, and Rock (1980) have analyzed the relationship between a direct and an indi rect measure of writing ability. Werts et al. assumed that the same true score underlies both measures and concluded that the test-retest reliability of the essay tests is biased due to correlated errors. The present analysis of their data shows that the direct and indirect tests measure two different abilities which correlate only .89 with each other and that it is not necessary to include correlated measurement errors for the essay tests. It is argued that the assumption that different tests measure the same ability should always be tested. Werts et al. (1980) did not test this assumption, and their conclusions, as a result, are incorrect.