Standardized tests are equated and scaled in order
that scores on different tests can be compared.
If one test yields higher expected scaled scores than
another, the scale is biased against those who take
the latter test. The amount of bias, defined as the
difference between expected values, depends on
ability. This paper presents two methods for estimating
this relationship and the bias in the scale,
using a predictor as the measure of ability. The resulting
evaluation is absolute in the sense that the
scale is judged according to its own properties and
not by comparison with an arbitrarily designated
criterion scale. Moreover, there is no need to assume
a particular theoretical model to be correct.
An application of the methods showed that the
Rasch model is not suitable for vertical equating of