This might kind of seem harsh, but I think it takes into account that there's a lot of knowledge to have in most subjects, and most people take very high levels of knowlege for granted. When I recruit, and when I train people on hiring, I propose a very curious rating on a zero to ten scale:
- don't know how to spell it
- heard of it
- wandering around the bookstore to find a book on it
- did some tutorials, half a clue
- used it a bit
- familiar with most stuff about it
- did a very substantial project with it
- multiple substantial projects with it; expert on my team
- expert at my company, or my region; contributed to the technology
- world expert that contributes to the technology world-wide; wrote or managed an implementation of this technology
- standards committte, inventor, owner of the technology
Most people are surprised at this because they rarely see experts who are better than 5 or so. They just don't think such peopl exist. But if you're hiring world-class talent, a big part of your team might be made up of people who are 7 or better on that scale.