Arcady's means value is incorrect in sampling in this margin. arcady is correct that self selection is not the most desirable method -- however, there is no methodology present that allows any other method. Which is to say that the argument is valid. The view is too narrow, though. The data's valid.

thou and I, my friend, can, in the most flunkey world, make, each of us, one non-flunkey, one hero, if we like: that will be two heroes to begin with. (Carlyle)