How to assess cognitive decline when test administration changes across study waves? Harmonizing cognitive scores across waves in the China Health and Retirement Longitudinal Study.

Background: Conducting longitudinal cognitive analyses is an essential part of understanding the underlying mechanism of Alzheimer's disease, especially for social and health behavior determinants. However, the cognitive test administration is highly likely to change across time and thus complicate the longitudinal analyses. The China Health and Retirement Longitudinal Study assessed memory through word recall tests across five study waves from 2011 to 2020. Since 2018, changes in the test stimuli and administration posed challenges for longitudinal cognitive analyses. Objective: To address differences in administration and to preserve differences attributed to characteristics such as age and education and to derive equated scores for use in longitudinal analyses in CHARLS. Methods: To ensure consistent underlying test ability across waves in the full sample (N = 19,364), we derived a calibration sample (N = 11,148) balancing age, gender, and education. Within this sample, we used weighted equipercentile equating to crosswalk percentile ranks between 2015 and 2018/2020 scores, then applied the algorithm to the full sample. Results: Mean original delayed word recall was higher in 2018 (4.3 words) and 2020 (5.1 words) versus 2015 (3.2 words). Following equating, scores in 2018 and 2020 aligned better with previous waves (2015, 2018, 2020 immediate means: 4.1, 3.6, 4.0; delayed: 3.2, 2.4, 2.9 words). Conclusions: Equipercentile equating enables the derivation of comparable scores, facilitating longitudinal analysis when cognitive test administration procedures change over time. We recommended the use of equated scores for longitudinal analyses using CHARLS cognitive data.