Individual differences make averaging meaningless in cancellation-level data

Evidence

Because there are large individual differences in such conditions, and because the minima are at different places, averaging is meaningless. All subjects show an increasing phase range for increasing frequency separation of the primaries. The subjects that show minima in the dependence of the cancellation level on primary levels also show phase steps at the same points. Other subjects show relatively smooth increments or decrements of the cancellation level, and also relatively smooth decrements of phase with increasing level of the lower primary. Further investigations have indicated that the correlation between the steps of phase and the minima in the cancellation level holds in all cases. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1584

Categorization

• QUESTION: Why do only some subjects show phase steps correlated with level minima, and what mechanism separates the two subject groups?