Nursing Project

I am wanting to find the minimum sample size that I need for my nursing project. I will be educating patients and the education will be the intervention. I want to know how many of the patients have initial knowledge on the topic, then I will educate, and then I will compare the pre and post test questionnaires to see the percentage of learning that occurred. The patients will be from a small internal medicine practice and for instance, my professor's sample size for her study was 27 patients. Thus, I want some sort of t-test and the G power for my project.