Stability of Rasch Scales Over Time

Item response theory (IRT) methods are generally used to create score scales for large-scale tests. Research has shown that IRT scales are stable across groups and over time. Most studies have focused on items that are dichotomously scored. Now Rasch and other IRT models are used to create scales for tests that include polytomously scored items. When tests are equated across forms, researchers check for the stability of common items before including them in equating procedures. Stability is usually examined in relation to polytomous items' central “location” on the scale without taking into account the stability of the different item scores (step difficulties). We examined the stability of score scales over a 3–5-year period, considering both stability of location values and stability of step difficulties for common item equating. We also investigated possible changes in the scale measured by the tests and systematic scale drift that might not be evident in year-to-year equating. Results across grades and content areas suggest that equating results are comparable whether or not the stability of step difficulties is taken into account. Results also suggest that there may be systematic scale drift that is not visible using year-to-year common item equating.