A study was done to investigate a newly created drug to increase memory performance. The study was done with rats. The dependent measure was number of errors made while learning a maze after being injected with the memory drug or a saline solution (control). Rats were randomly assigned to either the memory-enhancing drug or the control. A total of 30 rats was tested; there were 15 in each group. The mean (and standard deviation) for the drug group was 11.7 (4.7); that of the control group was 15.1 (5.1). (Lower numbers mean better performance.) What is the effect size for this study?