Objectives: (A) To measure the extent to which different candidate outcome measures identified highrisk prescribing that is potentially changeable by the data-driven quality improvement in primary care (DQIP) intervention.(B) To explore the value of reviewing identified high-risk prescribing to clinicians.(C) To optimise the components of the DQIP intervention. Design: Mixed method study. Setting: General practices in two Scottish Health boards. Participants: 4 purposively sampled general practices of varying size and socioeconomic deprivation. Outcome measures: Prescribing measures targeting (1) high-risk use of the non-steroidal antiinflammatory drugs (NSAIDs) and antiplatelets; (2) 'Asthma control' and (3) 'Antithrombotics in atrial fibrillation (AF)'. Intervention: The prescribing measures were used to identify patients for review by general practices. The ability of the measures to identify potentially changeable high-risk prescribing was measured as the proportion of patients reviewed where practices identified a need for action. Field notes were recorded from meetings between researchers and staff and key staff participated in semistructured interviews exploring their experience of the piloted intervention processes. Results: Practices identified a need for action in 68%, 25% and 18% of patients reviewed for prescribing measures (1), (2) and (3), respectively. General practitioners valued being prompted to review patients, and perceived that (1) 'NSAID and antiplatelet' and (2) 'antithrombotics in AF' were the most important to act on. Barriers to initial and ongoing engagement and to sustaining improvements in prescribing were identified. Conclusions: 'NSAIDs and antiplatelets' measures were selected as the most suitable outcome measures for the DQIP trial, based on evidence of this prescribing being more easily changeable. In response to the barriers identified, the intervention was designed to include a financial incentive, additional ongoing feedback on progress and reprompting review of patients, whose high-risk prescribing was restarted after a decision to stop.