Optimal assignment methods in three-form planned missing data designs for longitudinal panel studies

Planned missing designs are becoming increasingly popular, but because there is no consensus on how to implement them in longitudinal research, we simulated longitudinal data to distinguish between strategies of assigning items to forms and of assigning forms to participants across measurement occasions. Using relative efficiency as the criterion, results indicate that balanced item assignment coupled with assigning different forms over time most often yields the optimal assignment method, but only if variables are reliable. We also address how practice effects can bias latent means. A second simulation demonstrates that (a) assigning different forms over time diminishes practice effects and (b) using planned-missing-data patterns as predictors of practice can remove bias altogether.