Related Experiment Video
Updated: Jan 3, 2026

Subjective Refraction Test Using a Smartphone for Vision Screening
Published on: October 18, 2024
Even-number Measurement Bias With Goldmann Applanation Tonometry
Anne Rozwat1, Daniel K Roberts1,2,3
1Department of Clinical Education, Illinois Eye Institute, Illinois College of Optometry.
Abstract:
While working with a large data set of Goldmann applanation tonometry (GAT) intraocular pressure (IOP) measurements for a separate investigation, we observed that the IOP distribution favored even-numbered values. Study of this phenomenon has been limited, and this larger data set enabled inquiry that showed new insight concerning the manner in which this bias may vary according to IOP level. This analysis of 69,537 right eyes measured via GAT demonstrated that even-number preference was essentially absent in the 21 to 24 mm Hg range, but then the bias gradually strengthened as it moved away from this range in a lower or higher IOP direction. These data suggest that there is an examiner proclivity to measure and/or record GAT readings as even-numbered values, but that this tendency varies depending on IOP level.
Related Concept Videos
Pre-Procedural Guidelines for Assessing Blood Pressure
Errors in Taping
Common Leveling Mistakes and Errors
Special considerations while measuring blood pressure
Monitoring Both Arms:
Monitoring BP in both arms during the initial assessment is advisable, as the systolic value may differ by five to ten mm Hg between arms. For subsequent BP assessments, use the arm with the higher reading.
Uncertainty in Measurement: Reading Instruments
Errors occurring during blood pressure monitoring
Several factors...

