A successful week is when you win HoH/vote correctly.
An unsuccessful week is when you're one of the final nominees/you vote incorrectly.
Often times, those who have good success rates do very well in the game. I've been doing this for a little bit, and typically, the eventual winner of the season has a very good success rate. Obviously this statistic has a lot of flaws, but it's interesting to see who has been in the most control over the entire course of the season.