Using the same numbers as before, the average conversion rate is 0.5, the std-dev is 0.5, and thus you will need 25,600 users going through checkout to detect a 5% change based on 4∙2 ⋅ 0.5⋅(1−0.5) / (0.5 ⋅0.05) 2 . Since we excluded the 90% who do not initiate, the total number of users to the website should be 256,000, which is almost half the previous result, thus the experiment could run for half the time and yield the same power.