The authors supplied two pairs of spectacles to 43 patients with muscular imbalance and asthenopic complaints. Each had identical frame specifications and identical lens prescriptions; one had prism in an amount calculated by Sheard's criterion, the other did not. Neither the patients nor the authors knew which pair of spectacles had prism through the double blind experimental procedure. The lenses with prism base-out were preferable to patients but base-in was preferred only by presbyopes for near-point activities.
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Worrell et al. (1971) studied this question.