This study compared medication errors detected using incident reports, the Global Trigger Tool method, and direct observations of patient records. Incident reports and the Global Trigger Tool more commonly identified medication errors likely to cause harm. Omission errors were commonly identified by all three methods, but identification of other errors varied. For example, incident reports most commonly identified wrong dose and wrong time errors. The contributing factors also varied by method, but in general, communication issues and human factors were the most common contributors.