We are working out the details of the bisection method In  order to gain a ground truth against which the results of other measurements  can be checked. Bisection is in fact the oldest method of psychophysical scale  construction, which is, however, seldom used today. Its unique feature is that  the only mental operation which the subjects have to perform is to judge  whether the observed differences between two pairs in a triad of objects are  equal or not. Therefore a bisection experiment does not require the  investigator to make assumptions which are difficult to verify, e.g. about how  subjects form and apply categories or how they use numbers.