To Reject H 0 Correct Decision Type II Error Probabilities a is the probability of making a Type I Error and is called the level of significance or the a level. 1 - a is called the level of confidence. 1 - b is known as the power of a test. The power of a test is the ability of a statistical test to detect true effects when they exist. Thus, power is the probability that you will reject the null hypothesis when it is false, i.e., the probability that you will detect true differences when they exist.
Researchers can select the alpha level they wish to use.
Common alpha levels include .05 and .01. Choosing an Alpha Level Make a too large and you will commit too many Type I errors. Make a too small and you will not have enough power to detect true effects when they exist. Abuses of Statistical Tests Statistical inference is not valid for all sets of data. Don't overlook non-significance.