| Abstract: | For assessments that use different forms in different administrations, equating methods are applied to ensure comparability of scores over time. Ideally, a score scale is well maintained throughout the life of a testing program. In reality, instability of a score scale can result from a variety of causes, some are expected while others may be unforeseen. The situation is more challenging for assessments that assemble many different forms and deliver frequent administrations per year. Harmonic regression, a seasonal‐adjustment method, has been found useful in achieving the goal of differentiating between possible known sources of variability and unknown sources so as to study score stability for such assessments. As an extension, this paper presents a family of three approaches that incorporate examinees' demographic data into harmonic regression in different ways. A generic evaluation method based on jackknifing is developed to compare the approaches within the family. The three approaches are compared using real data from an international language assessment. Results suggest that all approaches perform similarly and are effective in meeting the goal. The paper also discusses the properties and limitations of the three approaches, along with inferences about score (in)stability based on the harmonic regression results. |
