Intraexaminer repeatability and agreement in stereoacuity measurements made in young adults

Abstract

Aim: To determine the repeatability and agreement of stereoacuity measurements made using some of the most widely used clinical tests: Frisby, TNO, Randot and Titmus.

Methods: Stereoacuity was measured in two different sessions separated by a time interval of at least 24h but no longer than 1wk in 74 subjects of mean age 20.6y using the four methods. The study participants were divided into two groups: subjects with normal binocular vision and subjects with abnormal binocular vision.

Results: Best repeatability was shown by the Frisby and Titmus [coefficient of repeatability (COR): ±13 and ±12s arc respectively] in the subjects with normal binocular vision though a clear ceiling effect was noted. In the subjects with abnormal binocular vision, best repeatability was shown by the Frisby (COR: ±69s arc) and Randot (COR: ±72s arc). In both groups, the TNO test showed poorest agreement with the other tests.

Conclusion: The repeatability of stereoacuity measures was low in subjects with poor binocular vision yet fairly good in subjects with normal binocular vision with the exception of the TNO test. The reduced agreement detected between the tests indicates they cannot be used interchangeably.

Keywords: agreement; repeatability; stereoacuity; stereopsis.

Figure 1. Bland-Altman plots of the repeatability of stereoacuity measurements taken in subjects with normal binocular vision

The plot for the Frisby had only three points: (20, 0) 48 subjects; (30, 20) 1 subject; and (30, -20) 5 subjects. The solid line represents the averaged difference between final session and initial session measures (MD: mean difference). Dotted lines indicate the lower and the upper 95% limits of agreement (coefficient of repeatability, COR=±1.96×SD) (Wilcoxon test P>0.05).

Figure 2. Bland-Altman plots of the repeatability of stereoacuity measurements taken in subjects with abnormal binocular vision

The solid line represents the averaged difference between final session and initial session measures (MD: mean difference). Dotted lines indicate the lower and upper 95% limits of agreement (coefficient of repeatability, COR=±1.96×SD) (Wilcoxon test P>0.05).

Figure 3. Bland-Altman plots of agreement between the tests used to measure stereoacuity in subjects with normal binocular vision

The solid line represents the averaged difference between final session and initial session measures (MD: mean difference). Dotted lines indicate the lower and the upper 95% limits of agreement (coefficient of agreement, COA=±1.96×SD) (Wilcoxon test P value).

Figure 4. Bland-Altman plots of agreement between the tests used to measure stereoacuity in subjects with abnormal binocular vision

The solid line represents the averaged difference between final session and initial session measures (mean difference, MD). Dotted lines indicate the lower and the upper 95% limits of agreement (coefficient of agreement, COA=±1.96×SD) (Wilcoxon test P value).

Figure 5. Mean stereoacuities recorded using the different tests in session 1 for the two groups of subjects (showing normal or abnormal binocular vision)

Error bars represent 1 SD.