Werts, Breland, Grandy, and Rock (1980) have
analyzed the relationship between a direct and an indirect
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.