StATS: What is a critical value? (February 22, 2008).
Someone wrote in asking about the difference between a p-value and a critical value. I define a p-value on my web pages already. The critical value is typically a percentile from a particular distribution or some function of that percentile that a statistic is compared to. So, for example, a Z-statistic is often compared to the value of plus/minus 1.96. The plus/minus 1.96 is the critical value and it represents the 2.5%ile and 97.5%ile of a standard normal distribution.
This page was written by Steve Simon while working at Children's Mercy Hospital. Although I do not hold the copyright for this material, I am reproducing it here as a service, as it is no longer available on the Children's Mercy Hospital website. Need more information? I have a page with general help resources. You can also browse for pages similar to this one at Category: Hypothesis testing.