From what I've read, GMAC doesn't disclose the details of the algorithms that drive their test. The Princeton Review
book (I'm just going from memory here so I may be wrong) said that they estimate that the test probably has like five different "bins", or five different difficulty levels.
If you think about it, it would be very difficult to have more than 4 or 5 difficulty levels, because "difficulty" is such a subjective thing.
i dont believe "difficulty" in this case is subjective.
i imagine that the "difficulty rating" is derived from the experimental questions and their success rate.
If that were true, you could have 10 or 20 "difficulty bins."