Okay, when deciding which are the "bad" letters, you have to factor in the language (English, French or even Chinese depending on your market). The simplest way is to flip over a dictionary, you can see that letters like Q and Z have relatively much fewer entries. Of course, for a more accurate count, we have to consider more commonly used words and proper names (like people and place names) which are not proportional to the total entries. For example, P may have more general entries than T but the latter could have more commonly used words.