Determining minimally important differences for the PDQ-39 Parkinson's disease questionnaire.

OBJECTIVE: To determine minimally important differences for dimensions of the PDQ-39, a 39-item Parkinson's disease questionnaire. A minimally important difference is defined as the smallest change between two scores that is subjectively meaningful to patients. Data on minimally important differences are essential for the calculation of sample sizes in trials and surveys. METHODS: We conducted a postal survey of randomly selected members of 13 local branches of the Parkinson's Disease Society, asking them to complete the PDQ-39 on two occasions, 6 months apart. On the first occasion respondents received the PDQ-39, demographic questions and a request to provide their name and address if they were willing to take part in the follow-up survey. After 6 months, we sent those who had agreed another copy of the questionnaire and also asked them to indicate how much change they had experienced since baseline in overall health and in each of the eight domains of the questionnaire. RESULTS: We calculated minimally important difference for each dimension and the index score for those reporting minor change since baseline. The minimally important difference varied across dimensions. CONCLUSIONS: These results indicate the minimum magnitude of change that should be sought when designing studies to evaluate change over time in Parkinson's disease. Since minimally important differences differ across dimensions, those designing studies in which sample size calculations are based on the PDQ-39 as an outcome measure should select the dimension which is the primary variable of interest.