PMC

Two options for sample size specification. If an individual sample size is selected you will obtain power estimates for that sample size only (left), whereas a range of sample sizes supplies a power curve (right).

Examples of three different levels of power, assuming a normal distribution. In each case, the null distribution is centered at 0 and the alternative is centered at , both with a variance of 1. A statistic threshold of 1.64 controls the Type I error rate at 5% for a one-sided hypothesis. In all cases, power is the area under the alternative distribution for statistic values larger than the threshold of 1.64. The left panel shows a high power example. The middle panel illustrates that if the mean of the distribution is exactly the threshold, the power is 50%. Lastly, the right panel shows a case where power is very low.