How is the specific letter distribution chosen in Boggle, Scrabble, etc.?

I have some ideas involving boggle or scrabble or some of those games but I have no idea how the letters are chosen. Is there some sort of scientific formula for the specific distribution of the letters in those games or is it just kind of chosen at random by guessing how many letters there should be? If there is a science to it, I would like to know what it is. Also, if there is a random picking thing, there had to be some sort of thought process otherwise we could end up with 11 Qu's and like 2 E's. How do they select which letters should be in boggle?