# 13: Power

• • Contributed by David Lane
• Associate Professor (Psychology, Statistics, and Management) at Rice University

Power is defined as the probability of correctly rejecting a false null hypothesis. For example, it can be the probability that given there is a difference between the population means of the new method and the standard method, the sample means will be significantly different. The probability of failing to reject a false null hypothesis is often referred to as β. Therefore power can be defined as:

power=1−β

It is very important to consider power while designing an experiment. You should avoid spending a lot of time and/or money on an experiment that has little chance of finding a significant effect.

• 13.1: Introduction to Power
Power is defined as the probability of correctly rejecting a false null hypothesis. In terms of our example, it is the probability that given there is a difference between the population means of the new method and the standard method, the sample means will be significantly different.  It is very important to consider power while designing an experiment. You should avoid spending a lot of time and/or money on an experiment that has little chance of finding a significant effect.
• 13.2: Example Calculations
• 13.3: Power Demo
• 13.4: Power Demo II
• 13.5: Factors Affecting Power
Several factors affect the power of a statistical test. Some of the factors are under the control of the experimenter, whereas others are not. The following example will be used to illustrate the various factors.
• 13.6: Statistical Literacy
• 13.E: Power (Exercises)