This demonstration shows various aspects of power. The graph displays power for a one-sample Z-test of the null hypothesis that the population mean is 50. The red distribution is the sampling distribution of the mean assuming the null hypothesis is true. The blue distribution is the sampling distribution of the mean based on the "true" mean. The default value of the true mean is 70. The cutoff for signficance is based on the red distribution. For a one-tailed test at the 0.05 level, the cutoff point is determined so that 5% of the area is to the right of the cutoff. For the default values, the cutoff is 58.22. Therefore, any sample mean ≥ 58.22 would be significant.

The shaded area of the blue distribution shows power. It is the probability that the sample mean will be ≥ 58.22 if the true mean is 70. For the default values it is 0.991.

You specify the true mean by either enterng text in the box. The populaton standard deviation can be specified by the pop-up menu. Finally, you cacn specify the number of tails of the test and the significance level. Power and the cut-off points are displayed.