We buy market data from two sources every year. They get their data by collecting it from the companies that buy their data. Obviously that's not perfect, but it's more objective than most other methods.
That hardest part is figuring out which generic categories match our specific positions, since every company in the survey has different definitions and requirements.
Our employees seem pretty happy with the results, and we've had very few offers rejected for salary range reasons. So it seems like we're pretty close. If anything, we're probably a bit above our local market.