<p>If we accept things like <a href="https://stats.stackexchange.com/questions/26/what-is-a-standard-deviation">that about standard deviation</a>  (upvoted 6 time, I think I have few questions that are so much upvoted :)) </p>

<p>I guess we can accept a question already asked in a book , if the book does not give the answer ...  The best you can do is editing the question (or ask to) with a reference to the book and an improvement with respect to the key definitions.</p>
