The World Glaucoma Association recommends that, to assess the effect on intraocular pressure (IOP) of medication or surgery, the mean of multiple measurements of postoperative IOP is a better reflection of IOP than a single measurement of postoperative IOP. The authors hypothesised that a single IOP measurement might approximate the mean of several measurements. Pre-randomisation data from the COMPASS study was analysed by the authors. A total of 609 patients (609 eyes) with primary open-angle glaucoma and cataract were included. A single randomly-timed IOP measurement was made at the screening visit while the patient was still receiving their usual glaucoma medication regimen. The medications were then discontinued (for pre-specified periods of time) and, at a baseline visit following complete medication washout, IOP measurements were taken at 8am, 12pm and 4pm. The authors calculate the proportion of eyes in which the increase in IOP after washout, using the mean of the three measurements, differed by more than 0.5, 1.0, 1.5, or 2.0mmHg from the increase in IOP after washout using only one of the post washout measurements. The mean (SD) IOP before washout was 18.5 (4.0) mmHg. The mean increase in IOP after washout, using the mean of the three measurements, was 5.3 (4.2) mmHg. The percentage of eyes in which the increase in IOP using a single post washout IOP differed from the increase in IOP using the mean of three measurements by more than 1.5mmHg was 35.1%, 25.6%, 34.2%, 30.0% and 31.4% when the single measurement was made at 8am, 12pm, 4pm, a randomly chosen single measure of those three times, and the time closest to that of the prewash out IOP, respectively. As more than 10% of the eyes (previously agreed cut-off) had a difference greater than 1.5mmHg for all single measurements, compared to the mean of three IOP measurements, the authors concluded that single IOP measurements are not a reliable replacement for multiple diurnal IOP measurements for IOP lowering medication, although they might be for post-surgical interventions, as the diurnal variation post-surgery is known to be narrower.