"Unfortunately, most people do not understand even the basic elements of statistics and probability, which is why astrologers and advertising agencies flourish. If you want to start an interesting fight, say in a loud voice at your next cocktail party; 'Fifty percent of Americans (or whatever) are mentally subnormal.' Then watch all those annoyed by this mathematical tautology instantly pigeonhole themselves." - Clarke - Credo
Of course, advertising agencies flourish, in part, because people want to obscure the statistical and mathematical realities which would normally be a component of "rational choice" decisions. Of which we are more capable than we give ourselves credit, I think.
As C. Pettit points out, George Carlin said the same thing, in his own inimitable fashion:
"Take a look around you and figure out how stupid the average person is...then realise that half the country is even stupider than that person...in other words, there are a hell of a lot of f***ing stupid people!!" - the immortal George Carlin - Complaints and Grievances
Bonus from Harlan Ellison: "The great art critic and philanthropist Bernard Berenson once said 'Consistency requires you to be as ignorant today as you were a year ago.'"