That’s the question that I was asked recently by one of the co-workers. It is simple and not so simple at the same time. It really depends on what you are looking for, what is the acceptable accuracy, and what is that you are comparing Google Analytics with.
How much will numbers differ? Here is what Google Analytics blog has to say:
Having used Google Analytics on a number of sites over a number of years, I’d say that that is just about right.