The question is, "What is the probability that the experimenter is going to be able to demonstrate that the null hypothesis is false by rejecting it at the .05 level?" This is the same thing as asking "What is the power of the test?" Before the power of the test can be determined, the standard deviation (σ) must be known. If σ = 10 then the power of the significance test is 0.80. (Click here to see how to compute power.) This means that there is a 0.80 probability that the experimenter will be able to reject the null hypothesis. Since power = 0.80, b = 1-.80 = .20.

It is important to keep in mind that power is not about whether or not the null hypothesis is true (It is assumed to be false). It is the probability the data gathered in an experiment will be sufficient to reject the null hypothesis. The experimenter does not know that the null hypothesis is false. The experimenter asks the question: If the null hypothesis is false with specified population means and standard deviation, what is the probability that the data from the experiment will be sufficient to reject the null hypothesis?