Free access. Despite consensus that preventing patient safety events is important, measurement of safety events remains challenging. This is, in part, because they occur relatively infrequently and are not always preventable. There is also no consensus on the ‘best way‘ or the ‘best measure’ of patient safety. The purpose of all safety measures is to improve care and prevent safety events; this can be achieved by different means. If the overall goal of measuring patient safety is to capture the universe of safety events that occur, then broader measures encompassing large populations, such as those based on administrative data, may be preferable. Acknowledging the trade-off between comprehensiveness and accuracy, such measures may be better suited for surveillance and quality improvement (QI), rather than public reporting/reimbursement. Conversely, using measures for public reporting and pay-for-performance requires more narrowly focused measures that favour accuracy over comprehensiveness, such as those with restricted denominators or those based on medical record review.