“…The mean time of starting to respond on a fixed-interval schedule of reinforcement is proportional to the interval from food to prime; the standard deviation of the start times is also proportional to this interval; thus, the coefficient of variation (the standard deviation divided by the mean) is relatively constant (Church, Meck, & Gibbon, 1994;Gibbon, 1977). The mean time of starting to respond also depends on the variability of the food-prime interval (Brunner, Fairhurst, Stolovitzky, & Gibbon, 1997;Church, Lacourse, & Crystal, 1998). The observation that animals respond sooner after reinforcement if the time to the next reinforcement is more variable may be related to the preference for variable over fixed times of reinforcement (for example, Davison, 1969;Gibbon, Church, Fairhurst, & Kacelnik, 1988).…”