Self-assessment is a process of interpreting data about one's performance and comparing it to explicit or implicit standards.To examine the external data sources physicians used to monitor themselves.Focus groups were conducted with physicians who participated in three practice improvement activities: a multisource feedback program; a program providing patient and chart audit data; and practice-based learning groups. We used grounded theory strategies to understand the external sources that stimulated self-assessment and how they worked.Data from seven focus groups (49 physicians) were analyzed. Physicians used information from structured programs, other educational activities, professional colleagues, and patients. Data were of varying quality, often from non-formal sources with implicit (not explicit) standards. Mandatory programs elicited variable responses, whereas data and activities the physicians selected themselves were more likely to be accepted. Physicians used the information to create a reference point against which they could weigh their performance using it variably depending on their personal interpretation of its accuracy, application, and utility.Physicians use and interpret data and standards of varying quality to inform self-assessment. Physicians may benefit from regular and routine feedback and guidance on how to seek out data for self-assessment.