to 5. Another possibility is that the presence of short intervals in the series somehow interfered with the ability to maintain a set over longer periods. However, this alternative explanation suffers from the absence of a statistically significant three-way interaction. In other words, one might have expected the deterioration in performance under the long Pis to have been more evident when there was a relatively high proportion of short Pis. REFERENCES KAELIN. L. Reaction time as a function of foreperiod duration and variability.