The paper presents a linking method between two different scales developed to measure the same construct, so that one of them may be the anchor to compare educational results over time. The Generalized Partial Credit Item Response Model is applied to multiple choice items, short answer, completion and open response items. The results suggest that scores are interchangeable. The statistical model is applied to obtain a common scale to allow the comparison of scores over time in Mathematics. It is applied to the area of educational assessment, but from the conceptual, methodological and operational point of view, it may be extended to other areas such as professional accreditation and certification, human resources evaluation and recruitment, health screening and diagnosis, psychological and behavioral evaluation. ; JRC.DDG.01-Econometrics and applied statistics