Screening for stereopsis without the use of technical equipment: scale development and cross-validation.

BACKGROUND Although adequate stereopsis is important for many common sensory-motor activities, large-scale normative samples of the relationship between stereopsis and other individual difference variables are relatively scarce, and stereopsis is often not routinely tested even in settings where it might play an important role. This is, in part, due to the fact that data collection requires individual testing, the use of costly technical equipment and trained personnel. METHOD Beginning with a pool of 161 items and an initial sample of 542 individuals, we developed a self-report inventory-suitable for group testing, survey administration or rapid individual screening--possessing a high correlation with laboratory measures of stereopsis. The inventory was then cross-validated against laboratory measures of stereo-acuity in a separate sample of 573 subjects. RESULTS For the combined samples, the resulting 10-item Stereopsis Screening Inventory (SSI) correlated with laboratory measures of per cent stereopsis (r = 0.80). The inventory's reliability was assessed with a resulting internal consistency coefficient (alpha) of 0.88. CONCLUSIONS The SSI provides a fast, valid and inexpensive measure of uncorrected stereopsis which can be used for rapid screening or epidemiological surveys. The correct classification rate for the SSI was 84% for a low fence requiring 65% stereopsis or better, and 81% for a high fence of residual stereopsis of 25% or less. A conversion equation, with inventory and scoring procedure is given in the Appendix.