~13% when it comes to reading, ~20% when it comes to quantitative tasks.
[1] http://en.wikipedia.org/wiki/Piagets_theory_of_cognitive_dev...
> a pretty good view of what average competence looks like.
How interesting - I bet you could tell some good (and informative/scary) stories. I'd buy the book.
Due to the decentralized nature of education in the US, there is higher variation in outcomes. This likely leads to greater illiteracy in the US than in other industrialized countries.
Example: Jakob Nielsen publishes research that shows "people on the web don't read." The sample content used to determine this? The list of tourist attractions in North Dakota.
Garbage in, garbage out.
Finally, the vast majority of text on the web around forms etc. is useless, poorly written, obtuse, abstruse. People have been indirectly trained to ignore it. It's not surprising that most users ignore that messages you took the effort to write.
If you run a startup or a company whose audience is early adapters you get a skewed view of the average level of competence of users.
I don't know if things get worse in other countries. However, I would guess that 10-20% of the US population lacks the basic literacy and logic skills to hold a manual job involving anything but repetitive tasks.