Research on frequency and severity of failures

Alastair wrote: In a typical audit we usually get 10-14 level-A issues, 
4-7 AA issues, is that the same for others?

When I was working on a model for accelerating accessibility, I had very 
similar questions not only about the relevance of WCAG levels, but the 
nature, frequency, severity and remediation efforts for accessibility 
issues. I've identified this as a research opportunity, as I did not find 
much useful data. I'd welcome any references, data sources or 
non-anecdotal findings on this topic.

Michael Gower