Statistical Power

The Probability that we will correctly reject the null hypothesis

Authors

Affiliations

Nate Yomogida, B.S., SPT

Doctor of Physical Therapy

B.S. in Kinesiology

Chloë Kerstein, B.S., SPT

Doctor of Physical Therapy

B.A. in Neuroscience

Overview

Definition

• Power is the probability that the study will give a significant result if the research hypothesis is true¹.
• In other words, power is the probability that we will correctly reject the null hypothesis.
• Power is the probability that we will correctly get a small p-value.

Power

Significance to the reader

• Important for making sense of results that are not significant¹
• Or results that are significant but not practically significant¹

Calculation

• We typically do not have the population mean, thus we must estimate it.

Factors influencing Power

• Sample size is directly related to power

Example

• Interactive example

Power Analysis

• Tells us how large the sample size is required to have sufficient power
• Power analysis can be used to avoid “p-hacking”

How to conduct a power analysis

• Step 1: Choose how much power you want
• Step 2: Determine significant threshold (α)
• Step 3: Estimate effect size (d)

References

1.

Aron A, Coups EJ, Aron E. Statistics for Psychology. 6th ed. Pearson; 2013.