To assess repeatability, horizontal heterophoria measurements were taken from 31 subjects on five occasions, each between 1 and 4 weeks apart.
Distance heterophoria was measured using a Maddox Rod, and near heterophoria was measured using a Maddox Wing. In addition, near measurements were repeated with a +3.00 D lens in front of each eye to place the targets at optical infinity. In each case, the standard deviation of the five values was taken as a metric of the test variability.
On average, the variability of the Maddox Rod was half of the Maddox Wing, both with and without the lenses.
This consistency suggests that the difference reflects the test conditions rather than optical differences in the target position.