Response process validity of the 9-item shared Decision-Making Questionnaire (SDM-Q-9) in a cognitive interview study with patients with cancer

The patient-reported, psychometrically evaluated, and frequently used 9-item Shared Decision-Making Questionnaire (SDM-Q-9) assesses shared decision-making processes in medical encounters. This study aimed to investigate SDM-Q-9's response process validity from the perspective of patients with cancer. We conducted qualitative cognitive interviews with N = 11 patients of a German comprehensive cancer center. Participants described their item interpretations, content anchors for highest and lowest scores, and difficulties with item rating. We performed qualitative content analysis and assessed concordance between item interpretations and the underlying theoretical construct of SDM-Q-9. Participants reported it necessary to have a choice (i.e., awareness of more than one option) to answer SDM-Q-9 and had difficulties rating one specific medical encounter only. For most items, interpretations matched the underlying theoretical construct. Discordance was found for item 2 (sharing the decision), and partial discordance for items 5 (supporting comprehension) and 6 (eliciting preferences). Content anchors for highest and lowest ratings showed substantial inter-individual variance. Results regarding SDM-Q-9's response process validity suggest validity for most items. However, reported prerequisites to be able to answer SDM-Q-9 and discordance between the underlying theoretical construct and interpretations found for three items warrant further investigation, before revision of the measure should be considered.