The PDQ-8: Development and validation of a short-form Parkinson's disease questionnaire

The Parkinson's Disease Questionnaire (PDQ-39) is a disease specific measure of health status which covers eight dimensions of ill-health, and contains 39 questions. The development of the questionnaire has been outlined in full elsewhere and is briefly outlined in this paper. Psychometric techniques are used here to derive a single index figure from the PDQ-39, transformed to a score range from 0 (good health) to 100 (poor health). A sub-set of items is then selected from the PDQ-39 to create a shorter form version, the PDQ-8. Each item selected is the one most highly correlated with the overall domain score to which it contributes: in cases where more than one item correlated equally strongly with the total then the item which received the highest completion rate was selected. The items were summed together and transformed onto a score from 0 to 100. The results gained from the single index gained from the PDQ-39 are compared with that gained from the PDQ-8, and are found to be remarkably similar. The use of the PDQ-8 is recommended over the PDQ-39 where a shorter form measure is required and where a single index measure of overall health status is acceptable or desirable.