Background and purpose: It is likely that the most common method for calculating a Roland Morris Disability Index (RMDQ) sum score is to simply ignore any unanswered questions. In contrast, the raw sum score on the Oswestry Disability Index (ODI) is converted to a 0-100 scale, with the advantage of allowing missing data to be accommodated by proportional recalculation. The aim of this study was to quantify the calculation error in RMDQ scores when one or more questions were unanswered and compare this with the error present when the ODI was scored in the same way. Methods and results: The prevalence of unanswered RMDQ questions was measured in a research and a routine care setting. The accuracy of the RMDQ proportional recalculation method was measured using 311 fully completed RMDQ and matching ODI questionnaire sets. Raw sum scores were calculated, and questions systematically dropped. At each stage, sum scores were converted to a score on a 0-100 scale and the error calculated. Wilcoxon Tests were used to compare the magnitude of the error scores. The prevalence of unanswered questions was 29.5% (RMDQ) in routine care, and 13.9% (ODI) and 20.3% (RMDQ) in a research project. Proportional recalculation was a more accurate method to calculate RMDQ sum scores than simply ignoring missing data. Conclusions: The practice of expressing RMDQ scores as a standardized score allows missing data to be accommodated by proportional recalculation and is as valid for the RMDQ as it is for the ODI.