I've done some research on the admissions411 site and here is how I would advise people to use the data: use it for school comparisons, but NOT for individual school stats.
For example using admissions411 data to answer questions such as "what is the average GMAT score for Darden applicants" or "what is the % of applicants invited to interview for Chicago" would be misleading. The reason for this is because the data is highly skewed in terms of GMAT score, since people with high scores will be more likely to post them. Thus Darden's GMAT would be like 730 and this would also help explain why Chicago's invite rate is close to 70%.
However, since the data is skewed for EVERY school, the irony is that because of this using admissions411 data to compare schools actually becomes valid (assuming there is no bias for any particular school, which I haven't found any). Hence using admissions411 data to answer questions such as "Which school has a higher average GMAT, Darden or Chicago" or "Which school would students prefer Wharton or Chicago" is perfectly valid.