Abstract: A method and apparatus for evaluating the performance of a device, such as a quadrature encoder, which generates at least two electrical wave train signals, each varying between first and second states and the two normally being out of phase one with the other. In the invention, four types of change of phase relation between the signals are distinguished from a succession of changes, the interval required for each type of phase change is timed and recorded, and a predetermined plurality of sets of records are accumulated. The mean time interval for a change of phase relation between the two wave train signals is computed from the accumulated plurality of sets of records and compared with the accumulated sets of records of timed intervals for determining the one type of phase change for which the difference from the mean time interval is the greatest.