I am planning a study to assess the correlation of perceived stress in medical students and their academic performance. After a literature review, it appears that one previously done recent study has reported a correlation coefficient value of 0.47. But when I use this value to calculate my required sample size through some online calculators, my N value comes out to be only 45.
Not sure if I'm doing something wrong, can someone kindly point me out to a standard formula for sample size calculation in correlational studies?
Best Answer
I would be wary of using the published value of r, 0.47, as the basis for your sample size calculations.
What if the true correlation is say 0.25? If that were the true population correlation, would you want your study to find a "significant" result? If so, compute the sample size for r = 0.25 (or even smaller). More generally, try to find the sample size that can detect (with reasonable power) the smallest effect (correlation coefficient for this example) you would care about.