A recent Pew Internet survey finds that 2% of US adults have a disability that makes using the Internet harder or impossible; 8% of people with disabilities report this problem. There’s some more detail and an online dialogue about the study. Pew will be releasing more reports soon.
Meanwhile, the UK Ofcom 2009 report on consumer experience finds that 7% of the total population, and about 20% of people with disabilities, have difficulty using a computer (which is not quite the same as the Internet, of course, but pretty close).
There may be many reasons for the disparity, but both studies are moving us closer to what industry and advocates need: a clear picture of how many people are disadvantaged by inaccessible technologies, down to a level of detail that will help designers and consumers.
By the way, another statistical tool that also helps frame the role of design decisions in inclusion is the aptly named Exclusion Calculator from the University of Cambridge Engineering Design Centre.
Update: the Ofcom 2010 Consumer Experience report shows an even disparity between disabled and non-disabled users regarding difficulty using a computer (see Table 206).