We handled missing data in accordance with the directions set out in the specific manual for each PRO.
Due to a low number of missing values we in accordance with Donner [ 24] decided to replace missing values of the CDS with mean values.
Calculation of the scores and treatment of missing data were done in accordance with the description at www.koos.nu.nu
The missing items were treated in accordance with the manual of the PRO in question in order to calculate the total score for the different PROs (Appendix Table 1, see Supplementary data).
Imputations for missing responses were carried out in accordance with the guidelines given by the developers of the questionnaire.
In accordance with the manual, missing items were imputed using the individual mean scores of items that were not missing in patients for whom at least 75% of items were available.
In accordance with the guidelines missing items in the STAI and PCQ scales were imputed by participants' own average score if at least 50% of these items had been completed (Ware et al, 1993).
Floor and ceiling effect was in accordance with other questionnaires, and missing data was limited to a range of 0-70-7%r all questions.
In accordance with developer instructions, if missing data for a SF-36 domain was ≤ 50% then data were imputed using the mean of participants' responses to the completed items [ 29].
