Jeff Sauro goes through a list of tips to help you sort and report your observations. He also provides some statistics to help you understand the differences between and relative value of testing methods.
Some of Jeff's advice from this article:
If you observe a problem in a usability test with a small sample size, it is more likely that this problem affects a lot of users than just a few.
Asking users to report what problems they encountered, where they were in the interface and potential solutions can reveal around half the problems as trained usability evaluators.
Five users will reveal most of the obvious problems not all problems.
10 Things to Know about Usability Problems - article by Jeff Sauro