I just did a steady grade hill sprints workout. Chances are, sprints are not going to be as accurate as a long steady, lower wattage effort. While my power meter (SRM) was consistantly averaging 700+ watts over 30+ seconds, Analytic cycling predicts 590 watts. Does this mean it is no good? Nope. I don't know the exct air density or what my drag coefficient was, etc. If you know all of those numbers, then great, otherwise, you should take it as a rough estimate.