Number Needed to Screen

1. See Also
   1. Screening Test
   2. Contingency Grid (includes Statistics Example)
   3. Test Sensitivity
   4. Test Specificity
   5. Negative Predictive Value
   6. Positive Predictive Value
   7. Likelihood Ratio
   8. Length Bias
   9. Leadtime Bias
   10. Selection Bias
2. Definition: Event Rate (Event Probability)
   1. Event Rate: (Number Events) / (Number Total Patients)
   2. Calculate for both intervention and control groups
3. Definition: Relative Risk (RR)
   1. RR = (Intervention Event Rate)/(Control Event Rate)
4. Definition: Relative Risk Reduction (RRR)
   1. Relative Risk Reduction or RRR = 1 - (Relative Risk)
   2. RRR = (Absolute Risk)/(Control Group Event Probability)
5. Definition: Absolute Risk Reduction (ARR)
   1. ARR = (Intervention Event Rate) - (Control Event Rate)
6. Definition: 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)
   4. NNT = 100/abs(rateTreatment - rateControl)
      1. Where Intervention is intended to prevent event X (e.g. Myocardial Infarction)
      2. Where abs = Absolute Value
      3. Where rateTreatment = Rate of event X in treatment group (e.g. 5%)
      4. Where rateControl = Rate of event X in a control group (e.g. 10%)
      5. Example: NNT = 100/abs(5-10)=20
         1. Twenty persons would need to undergo intervention, to prevent one event X
7. Resources
   1. Visual Tool
      1. http://www.nntonline.com
8. References
   1. Cook (1995) BMJ 310:492-4
   2. Daya (1999) Evid Based Obstet Gynecol 1:103-4
   3. Mcquay (1997) Ann Intern Med 126(9):712-20