The Pulsair 3000 tonometer--how many readings need to be taken to ensure accuracy of the average?

Manufacturers of non-contact tonometers recommend that a number of readings are taken on each eye, and an average obtained. With the Keeler Pulsair 3000 it is advised to take four readings, and average these. This report analyses readings in 100 subjects, and compares the first reading, and the averages of the first two and first three readings with the "machine standard" of the average of four readings. It is found that, in the subject group investigated, the average of three readings is not different from the average of four in 95% of individuals, with equivalence defined as +/- 1.0 mmHg.