The epidemiology of medical error has been intensively investigated in prior studies, with the goal of quantifying the overall burden of preventable harm on a population basis. This study uses econometric techniques to address a related but distinct question: what is the daily risk of a patient safety event for a typical hospitalized patient? The authors arrive at estimates of the daily risk of various safety problems, including adverse drug events, pressure ulcers, and hospital-acquired infections, and also find that the probability of experiencing an adverse event increases with increasing hospital length of stay.