An act of commission (doing something wrong) or omission (failing to do the right thing) that leads to an undesirable outcome or significant potential for such an outcome. For instance, ordering a medication for a patient with a documented allergy to that medication would be an act of commission. Failing to prescribe a proven medication with major benefits for an eligible patient (e.g., low-dose unfractionated heparin as venous thromboembolism prophylaxis for a patient after hip replacement surgery) would represent an error of omission.

Errors of omission are more difficult to recognize than errors of commission but likely represent a larger problem. In other words, there are likely many more instances in which the provision of additional diagnostic, therapeutic, or preventive modalities would have improved care than there are instances in which the care provided quite literally should not have been provided. In many ways, this point echoes the generally agreed-upon view in the health care quality literature that underuse far exceeds overuse, even though the latter historically received greater attention. (See definition for Underuse, Overuse, Misuse.) In addition to commission vs. omission, three other dichotomies commonly appear in the literature on errors: active failures vs. latent conditions, errors at the sharp end vs. errors at the blunt end, and slips vs. mistakes.