Survey researchers often use the recency question format - ''When was
the last time you...'' - to estimate purchase or usage rates. This for
mat is particularly useful when the behavior in question is of a sensi
tive nature. The calculation of interpurchase times and, in turn, rate
s from recency format responses requires an a priori assessment of the
distribution of interpurchase times. Although the usual assumption is
that interpurchase times are exponential, evidence indicates that tim
es between purchases are more regular. We relax the exponential assump
tion and measure the effects of more regular purchase behavior, discre
te purchase behavior and the dead period between purchases on estimate
d rates. Even though our focus is on recency, our results also give so
me insights into the commonly reported frequency based result that sho
ws heavy consumers are 'more regular' in their purchasing behavior.