AbstractThere has been much criticism of experimental work involving the teaching of signs to the mentally handicapped. Aside from poor experimental design and inadequate description of subjects, the presentation of results has been vague and the measurement problems involved in sign language research have not been formally addressed or solved. These problems lead to difficulties in replicating work and designing follow‐up studies. The current paper examines problems involved in attempts to measure sign acquisition and examines possible solutions in terms of reliability and validit