Measuring luck

[johnzzz]

I usually post on the Beginner's forum but I really want to nail this problem.

You play 100 hands of limit holdem and you've got basic PT type stats.

If you end up down by 20bb how can you tell why?

This question then leads on to loosing runs. In posts people say that its just variance, but how do they know? how can it be measured?

What are the stats(like in PT)you could use to measure?