I want to conduct a usability study for a software programming tool. There are two versions of the tool (prototype A and prototype B) with different features, but allowing the same goals. I want the participants to complete a set of tasks using both versions of the tool and report the time taken. I'll compare the time taken by the two methods. The same set of participants perform tasks using prototype A and then prototype B. How can I find out the number of participants required for the study?
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I would recommend the G*Power package which give multiple options to perform power calculations.
In your case it is likely that you use a paired t-test. So therefore you have to specify the following:
Hit calculate and you get the required sample size (34 in case of two tailed, an effect size of .5, an alpha of .05, and a power of.80)