How good is the estimated cover test at predicting the von Graefe phoria measurement?

Results obtained on the estimated cover test and the von Graefe phoria measurement were compared in order to determine how well the former test predicted the result of the latter. Subjects for the study were 25 clinic patients and 28 optometry students. Examiners were two fourth-year optometry interns. For testing at 6 m, correlations between the estimated cover test and the von Graefe measurement were relatively high, although for some subjects the cover test underestimated the von Graefe measurement by as much as 4 delta of exophoria and 6 delta of esophoria. For testing at 40 cm, correlation coefficients were somewhat lower, and the cover test underestimated the von Graefe measurement by as much as 11 delta of exophoria and 13 delta of esophoria. It was concluded that although the estimated cover test is valuable as a screening test, it should be followed by prism neutralization or by von Graefe phoria measurement.