Question: I know its a bit of a stupid question, but I started doubting myself yesterday. I need to find the expected 3 month returns, but I have weekly data. Was told that we need to assume that there are 12 weeks in 3 months, so I used (12th week/1st week)-1. However, my friend pointed out that we might need to use (13th week/1st week)-1 because otherwise we will loose one weekly period. The argument was that if you want to find weekly return you would do (2sd week/1st week)-1. so in order to find the ruturn for 12 weeks, you need (13th week/1st week)-1. I am confused now, what would be the right way to find the return?