Even-number Measurement Bias With Goldmann Applanation Tonometry.

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.