=+4 Understanding type I and type II errors is essential for a surgeon’s quality planning. In appendectomy operations, for example, removal of the appendix is necessary because of the risk of it bursting, causing potentially fatal poisoning of the blood. The surgical procedure is a relatively simple operation, but there is always a small risk with any invasive surgery. It is also expensive; in the USA it costs around $4,500 per operation. Unfortunately, appendicitis is difficult to identify, and diagnosis is only 10 per cent accurate. However, a new technique claims to be able to identify 100 per cent of true appendicitis cases prior to surgery. The new technique costs less than $250, which means that one single avoided surgery pays for around 20 of these tests.