Preparation of ambiguous stimulus materials

The problem of ensuring that ambiguous figures are equally probable in the appearance of each of their two alternative and mutually exclusive aspects is discussed. Failure to consider this problem raises difficulties for interpretation of the results of experiments in which such figures are used. A method of preparing and evaluating the extent to which ambiguous figures are equivocal according to acceptable criteria is described and illustrated. This yields a new ambiguous figure, each alternative aspect of which becomes apparent with approximately the same frequency.