Digit preference in Goldmann applanation tonometry: The hedgehog effect

PURPOSE: Digit preference is a subconscious bias towards numbers that end in certain digits. We conducted a study to assess for digit preference in intraocular pressure measurement using the Goldmann applanation tonometer. DESIGN: Prospective study with masked and unmasked phases. METHODS: The study was conducted in general ophthalmic clinics in a District General Hospital in Blackpool, United Kingdom, by 6 clinical ophthalmologists. Consecutive right eye intraocular pressure measurements were collected for a 2-month period for the masked phase and were repeated for the unmasked phase. The proportion of even numbers was the main outcome measure. RESULTS: Six ophthalmologists completed both phases: 555 of 917 readings were even in the masked phase (P = .075), and 679 of 1056 readings were even in the unmasked phase (P =.022). CONCLUSIONS: Our results suggest that digit preference influences measurements from the Goldmann applana, tion tonometer and that this persists when observers are aware of digit preference. This bias may influence both clinical decision-making and trial results. (Am J Ophthalmol 2005; 140:527-529. (c) 2005 by Elsevier Inc. All rights reserved).