Critical values are the values that indicate the edge of the critical region. Critical regions describe the entire area of values that indicate you reject the null hypothesis. In other words, the critical region is the area encompassed by the values not included in the initial claim – the area of the ’tails’ of the distribution. The “tails” of a test are the values outside of the critical values. In other words, the tails are the ends of the distribution, and they begin at the greatest or least value included in the alternative hypothesis (the critical values).