Voluntary event reporting systems are a mainstay of efforts to identify system failures that expose patients to harm. In order to improve the utility of event reporting systems, AHRQ developed the Common Formats for defining and classifying patient safety events. The Common Formats are designed to be used by Patient Safety Organizations for analysis of aggregated safety events. This study sought to evaluate the ability of nurses to use the Common Format definitions of harm severity. The investigators found relatively poor interrater reliability between nurses when asked to rate the severity of harm experienced by patients in several error scenarios (such as a medication error and a health care–associated infection). This implies that voluntary error reports may not accurately estimate harm, even when using a standardized reporting format. A prior study also found similar results.