A researcher collects a sample of individuals' productivity by measuring the number of words they typed in an hour during a writing session with 20 individuals. The sample mean is 1738.2 words this week. The individuals from last week's 20 individuals only had a mean of 1553.7 words. The pooled variance is 11545.9.
Calculate the Cohen's d for this mean difference.
A) .02
B) 1.7
C) insufficient information
D) -.02

Question

Quizplus · Accepted Answer

To calculate Cohen's d, we need to find the standardized mean difference between the two samples. Cohen's d = (M1 - M2) / pooled standard deviation, where M1 is the mean of the current sample, M2 is the mean of the previous sample, and the pooled standard deviation is the square root of the pooled variance (11545.9) divided by the sum of the sample sizes (40-2 = 38), which is approximately 34.07. Plugging in the numbers, we get (1738.2 - 1553.7) / 34.07 = 0.5423. Since this is greater than 0.2, the effect size is considered moderate to large according to Cohen's guidelines. Therefore, the answer is B.

A Researcher Collects a Sample of Individuals' Productivity by Measuring