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)

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Quizplus · Accepted Answer

Cohen's d is calculated as the mean difference divided by the standard deviation. Given a mean difference of 6.2 and a standard deviation of 8.4, Cohen's d = 6.2 / 8.4 = 0.738, which is approximately 0.74. This is considered a medium effect size according to Cohen's conventions.

A Researcher Computes the Mean Difference in Locomotion in a Sample