what is a critical value
I searched Google under the key words "statistics critical value" to get these possible sources:
http://www.google.com/search?client=safari&rls=en&q=statistics+critical+value&ie=UTF-8&oe=UTF-8
In the future, you can find the information you desire more quickly, if you use appropriate key words to do your own search. Also see http://hanlib.sou.edu/searchtools/.