A Researcher Computes the Mean Difference in Locomotion in a Sample

Multiple Choice

A researcher computes the mean difference in locomotion in a sample of 12 rats before and 30 min after an injection of amphetamine.Rats were placed in a box with infrared beams.The number of times rats crossed the beams was used as a measure of locomotion.The mean difference in locomotion was 6.2 ± 8.4 (M_D ± S_D) and this difference was significant.What is the effect size for this result using estimated Cohen's d?

A) d = 0.74 (medium effect)
B) d = 1.36 (medium effect)
C) d = 0.74 (large effect)
D) d = 1.36 (large effect)