These results surprised me. I expected a much wider gap in correct move % between a 1500-player and grandmaster. It'd be interesting to see if the slope of the graph is steeper for minor blunders that reduce the evaluation by less than a pawn. These are the more subtle positional errors - weakening a square, not maximizing piece activity, wrecking your pawn structure, etc. Amateur games are filled with these mistakes, but they are much rarer in GM games, and I'd expect the difference to be more than just a few percentage points. But Crafty's not the right engine for this job. You'd want something with a more sophisticated evaluation function, like Stockfish (several-hundred ELO stronger than Crafty).
Great! After thinking about it some more, I think I understand why the graphs are flatter than I expected. There are differing degrees of difficulty in tactical mistakes. When a 1500-player blunders a pawn or piece, it's often resolved by a trivial one-move sequence. GM blunders are more subtle, often requiring a lengthy (say 5-10 ply) sequence to resolve. You could prove this by recording the minimum search depth the engine needs to recognize the blunder. (This is tricky, because search extensions result in many sub-variations being analyzed much deeper than the nominal search depth, but I seem to recall that Crafty has an option for disabling extensions.)
