From the proportion matrix obtained by the method of paired comparisons, a mean point of subjective equality (PSE) may be computed for each stimulus. The almost linear function that relates mean PSE to stimulus size is called the mean PSE function. There are two mean PSE functions. For pairs of successive stimuli, one of these functions is relative to the stimuli that appear first and one to those that appear second in a pair. Because of time errors, the mean PSE functions intersect. The experimental results reported here seem to indicate that the parameters of the mean PSE functions depend on distinct response biases. The angle between the mean PSE functions seems to depend on the greater weight of the relative magnitude of stimuli that appear second in a pair. This angle also necessarily depends on the stimulus range. The relative vertical displacement of the mean PSE functions seems instead to be caused by interpolated stimuli and by the Woodrow effect. A response bias interpretation of time errors is offered.