1.1 ,This practice describes procedures to estimate Thurstonian discriminal distances (that is, d', values) from data obtained on two samples. Procedures are presented for four forced-choice methods (that is, the triangle, the duo-trio, the 3-alternative-forced-choice (or 3-AFC) and the 2-AFC (also called the directional difference test)), the A/Not-A method, the Same-Different method and for data obtained from ordered category scales. Procedures for estimating the variance of d', are also presented. Thus, confidence intervals and statistical tests can be calculated for d',.