Rasch modelling to deal with changes in the questionnaires used during long-term follow-up of cohort studies: a simulation study.

The use of the standardised score to deal with questionnaire changes over time was more efficient than the raw score and also, surprisingly, than the latent variable score. LCGA applied to the raw score was the least efficient and exhibited the most unbalanced misclassifications across trajectory groups. As prospective longitudinal studies with long-term follow-up are more and more common, researchers should be aware of this phenomenon and should reconsider the use of the raw score when changes in the questionnaires used occurred during follow-up.