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Number Needed to ScreenAka: Number Needed to Treat, Absolute Risk Reduction, Relative Risk Reduction

Event Rate (Event Probability)
1. Event Rate: (Number Events) / (Number Total Patients)
2. Calculate for both intervention and control groups
Relative Risk (RR)
1. RR = (Intervention Event Rate)/(Control Event Rate)
Relative Risk Reduction (RRR)
1. Relative risk Reduction or RRR = 1 - (Relative Risk)
2. RRR = (Absolute Risk)/(Control Group Event Probability)
Absolute Risk Reduction (ARR)
1. ARR = (Intervention Event Rate) - (Control Event Rate)
Number Needed to Screen (NNS) or Treat (NNT)
1. Number of patients screened to prevent one death
2. Assumes certain period of time (e.g. one year)
3. NNS or NNT = 1/(Absolute risk reduction)
Resources
1. Visual Tool
  1. http://www.nntonline.com
References
1. Cook (1995) BMJ 310:492
2. Daya (1999) Evid Based Obstet Gynecol 1:103-4
3. Mcquay (1997) Ann Intern Med 126(9):712

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This page was written by Scott Moses, MD. This page was last revised before 5/10/08, prior to consistent tracking of revision date. This page was last published on 5/10/2008. Please send comments and questions to the editor. Contact information may be found on the page About the Family Practice Notebook. These pages are best viewed with Microsoft Internet Explorer 6.0.