This is a pretty old conversation and we have upgraded the tests twice (GUI, Algorithm, and Questions too) since this has taken place.
Here is what I can share as of July 2016:1. The quant algorithm is extremely accurate. No, we do not know the official GMAT Algorithm (nobody does) and no we do not have the official questions, and yes, our questions are harder, so you can have more mistakes - seems like this would be a recipe for disaster for evaluating user ability. Kind of - it definitely would if we did not have Bunuel and user feedback. Bunuel has been involved in the last few algorithm revisions and helped building and tweaking it. We use a correlation method to adjust our algorithm based on the GMAT scores that users get and report back to us. This is an automated process that runs every few months. Each test score goes through a 4-stage grading process to ensure the score is most accurate, and it almost always is.
2. We have also made our free tests into CAT's as of March 2016. Previously these were fixed-question non-adaptive tests. Currently they are adaptive and use the same scoring algorithm as the paid tests.
3. Verbal algorithm has gotten an overhaul in March 2016 to improve accuracy. It was having challenges with accuracy due to a smaller verbal question database. We have modified the algorithm to be meaningfully different from Quant and as the result it is more accurate now.... though still not as accurate as quant.
4. You can see/read some of the anecdotal feedback/reviews from users on accuracy of the tests based on the reviews:
https://gmatclub.com/reviews/comments/gm ... -345351466https://gmatclub.com/reviews/comments/gm ... -345351397You can see all of the reviews here:
https://gmatclub.com/reviews/tests-gmat- ... -345345662Let me know if you have any questions about the scoring results.
BB