Hi, sorry not to have replied sooner; I was out of the country for a few weeks.

Our tests have a standard deviation of about 50 points and the official test has a standard deviation of about 30 points. Couple of take-aways here:

1) To have a standard deviation of 30 points means that if you score, say, a 650, and take the test again right away, you have a 2/3 chance of scoring between a 620 and a 680 and a 1/3 chance of scoring below a 620 or above a 680. There isn't a statistically significant difference between the scores within one standard deviation (so, in this example, if you score a 650, there's not a statistically significant difference between that score and any other score between 620 and 680). A pretty wide range - the official test is not as "accurate" as most people think it is.

2) With our standard deviation of 50, we are fairly close to the official test but we will never be able to hit the same standard deviation or a better (read: smaller) one (nor will any other test prep company) because it's impossible to create something exactly equal to or better than the official test.

3) I don't know what the standard deviations are for other companies' tests. But we actually publicize ours because we think ours is pretty good.