• The probability of saying or concluding that a study found no difference between groups when in reality there is a difference.
• This is also known as the chance of making a type II error or false negative rate.

• β ("Beta") is an important part of power, which is defined as being 1 - β.
• Most clinicians accept a power of 0.8 (or 80%), which means β has to be 0.2 (or 20%)
• Reducing β will decrease the chance of making a type I error and at the same time increase the power of the study.
• Note:  As β is decreased, the sample size will increase in order to find a difference if one exists. This will clearly add to the cost and time to complete a study.

• Alpha Value (α)
• P-value
• Power (1 - β)
  

Editors & Reviewers:

• Anthony J. Busti, MD, PharmD, FNLA, FAHA
  

Last Reviewed:  July 2015