A researcher conducts a t test for dependent means in which it is predicted that there will be a decrease in unemployment from before to after a particular job-skills training program. The cutoff "t" needed is -1.8333. The standard deviation of the distribution of means of change scores is 2.0 and the mean change score for the sample studied is an increase of 5.2.
What is the effect size?
- 5.2/-1.833 = -2.84; large
- 2/5.2 = .38; approximately medium
- 5.2/2 = 2.60; large
- It can not be determined without also knowing the population standard deviation
To calculate the effect size for the given scenario, we can use Cohen's d formula. Cohen's d is calculated by dividing the mean difference between two groups by the standard deviation of the population or sample. In this case, the mean change score for the sample studied is an increase of 5.2 and the standard deviation of the distribution of means of change scores is 2.0.
The formula for Cohen's d is:
d = (mean difference) / (standard deviation)
Using the given values, we have:
d = 5.2 / 2.0
d ≈ 2.60
Therefore, the effect size is approximately 2.60, which is considered large according to the effect size guidelines suggested by Cohen.
So, the correct option is:
- 5.2/2 = 2.60; large