Jakob Nielsen:

If you want to target a broad consumer audience, it’s safest to assume that users’ skills are those specified for level 1. (But, remember that 14% of adults have even poorer skills, even disregarding the many who can’t use a computer at all.)
 To recap, level 1 skills are:
 Little or no navigation required to access the information or commands required to solve the problem
 Few steps and a minimal number of operators
 Problem resolution requiring the respondent to apply explicit criteria only (no implicit criteria)
 Few monitoring demands (e.g., having to check one’s progress toward the goal)
 Identifying content and operators done through simple match (no transformation or inferences needed)
 No need to contrast or integrate information
 Anything more complicated, and your design can only be used by people with skills at level 2 or 3, meaning that you’re down to serving 31% of the population in the United States, 35% in Japan and the UK, 37% in Canada and Singapore, and 38% in Northern Europe and Australia. Again, the international variations don’t matter much relative to the big-picture conclusion: keep it extremely simple, or two thirds of the population can’t use your design.