Errors in medication administration are common and are generally considered to account for a significant proportion of all medication errors. However, this systematic review identifies an underlying flaw inherent in studies of medication administration errors: variations in the methods used to measure errors can result in widely differing estimates of error rates. For example, only 6 of 16 included studies considered administration of a drug at the wrong time to be an error. As delayed drug administration is relatively common, studies that did not include such wrong-time errors may have reported spuriously low error rates. The authors provide a series of recommendations for standardizing measurement and reporting of medication administration error rates, with the goal of generating studies that can be used for comparisons between institutions and across time periods. Prior studies have also raised concern that variations in the definition of medication errors can affect measured error rates, and medication error measurement standards have been developed for medication reconciliation studies.