How to calculate a players edge

[Hosayif]

Here's my situation: I play in a really cheap NL tourny in college with friends and their friends where i would say about 25% have practically never played before, 50% have played but are really bad, and 25% know what they are doing.

I was wondering, without having a lot of results to look back on, what methods do you guys use to approximate your edge in a game. Also, given ample data, how do you calculate your edge? Is it just your ROI?