A study found that rats that received a drug took longer to find their way out of a maze than rats that received a placebo, F(1, 47) = 5.29, p < 0.05. The drug had a large effect on albino rats and virtually no effect on wild-bred rats, F(1, 47) = 9.31, p < 0.05. However, there was no overall difference in the time of albino rats compared to that of wild-bred rats, F(1, 47) = 2.27, p > 0.10. Which effect(s) must be present for these results to be true?