I have a patient group and a control group. Each group will take a medical test. I would like to estimate the required sample size (power 0.8, alpha 0.05) if the effect size (cohens d) between groups (as pertains to between group differences for results on this test) is 0.5.
This part is straightforward - and I would use a program like G*power to perform the calculation.
The issue that I am unclear about is how to factor in the reliability of the test? For example I have data that the intraclass correlation coefficient obtained in test-retest study is 0.6.It seems clear that a less reliable test will have a lower chance of detecting group differences but I am unsure how to correctly quantify this.
Any help would be much appreciated