PURPOSE: The aim of this study was to reduce respondent burden and decrease data errors in patient-reported outcomes (PROs) intended for a postal survey measuring health and lifestyle factors that may affect quality of life in older people with multiple sclerosis (MS). METHODS: Participants (n = 18) were recruited from a database of outpatient visits. Using the qualitative diagnostic method, cognitive debriefing, participants completed five standardized questionnaires; Frenchay Activities Index, Barthel Index, Simple Lifestyle Indicator Questionnaire, EuroQoL EQ-5D and Personal Resources Questionnaire 2000. PRO item issues and respondent behaviors such as skipping items were recorded. Data collection was an iterative process whereby difficulties experienced by 2-3 subjects were used to modify the survey for following respondents until data saturation was reached. RESULTS: Most respondents had serious difficulties with at least one PRO item. Response errors fell into three main categories: (1) respondents did not read instructions and completed the item incorrectly, (2) respondents did not understand the question and required examples to clarify and (3) respondents felt that the pre-determined response options did not apply to them. PRO reformatting, minor modifications to item wording and addition of item examples improved precision and reduced respondent burden. CONCLUSION: Our findings support the notion that methods such as cognitive debriefing help improve precision of self-reported measures in a special population such as ours.