I'm looking to do a pre-posttest RCT comparing difference of means through a t-test, however I'm completely unsure how to calculate a required sample size using power, confidence level, and standardized effect size.
Then using this d value I can use a table to find the appropriate sample size. However, my main issue is determining the study to derive the X1 and X2 means in the formula from. Firstly, I'm assuming the test used to measure needs to match up (I.E. The study that I'm deriving the pre and post test means from need to use the same test as my trial, such as a VAS measure for pain) and obviously they both need to look at the same condition (Osteoarthritis in my case). However, does the study that I'm using the means from need to be completed on the same population and does the intervention need to be be the same dosage and duration or can a very similar dosage and duration be considered adequate?
Also, does the fact that I'm using an active control rather than a placebo have any influence on my calculation? How would modifying my trial to become a crossover design influence my sample size?
Thanks in advance, appreciate any support you guys can give me.