Mean (± SEM) estimates of Herrnstein's hyperbola parameters k (asymptotic target response rate; left panels) and R
(rate of reinforcement of non-target behavior; right panels) for each strain in epochs 1 and 2 (top and bottom panels, respectively). Estimates are based on hypotheses selected according to AICc (Table 1). Estimates of k for SHR were intermediate relative to other strains. Estimates of R
for SHR were low relative to other strains, and approximately constant across epochs.