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## Number Needed to Screen

Aka: Number Needed to Screen, Number Needed to Treat, Absolute Risk Reduction, Relative Risk Reduction
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)
3. Relative Risk and Relative Risk Reduction can be misleading when applied to an individual patient
1. Relative Risk Reduction reflects the effect of an intervention on a population
2. Effect is not guaranteed for the individual patient
3. Number Needed to Treat (NNT) is a better measure of the chance that a positive effect will apply to the individual patient
4. http://www.thennt.com/thennt-explained/
5. Definition: Absolute Risk Reduction (ARR)
1. ARR = (Intervention Event Rate) - (Control Event Rate)
2. A negative result is seen with effective intervention, while a positive result is seen with a harmful intervention (worse than control)
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 rateTreatment = Rate of event X in treatment group (e.g. 5%)
3. Where rateControl = Rate of event X in a control group (e.g. 10%)
4. Example: NNT = 100/(5-10)=-20
1. Twenty persons would need to undergo intervention, to prevent one event X
2. In this case, the result is negative, consistent with Number Needed to Treat
3. If the result were positive, the finding would be consistent with number needed to harm
7. Resources
1. Visual Tool
1. http://www.nntonline.com
8. References
1. Cook (1995) BMJ 310:492-4 [PubMed]
2. Daya (1999) Evid Based Obstet Gynecol 1:103-4
3. Mcquay (1997) Ann Intern Med 126(9):712-20 [PubMed]

### Relative Risk (C0242492)

Definition (NCI_NCI-GLOSS) A measure of the risk of a certain event happening in one group compared to the risk of the same event happening in another group. In cancer research, risk ratios are used in prospective (forward looking) studies, such as cohort studies and clinical trials. A risk ratio of one means there is no difference between two groups in terms of their risk of cancer, based on whether or not they were exposed to a certain substance or factor, or how they responded to two treatments being compared. A risk ratio of greater than one or of less than one usually means that being exposed to a certain substance or factor either increases (risk ratio greater than one) or decreases (risk ratio less than one) the risk of cancer, or that the treatments being compared do not have the same effects.
Concepts Quantitative Concept (T081)
MSH D012306
English Relative Risks, Risk, Relative, Risks, Relative, RELAT RISK, relative risk, Relative Risk, risk ratio
French Risque relatif
German Relatives Risiko
Italian RR, Rischio relativo
Norwegian Relativ risiko
Dutch Risico, relatieve
Sources
Derived from the NIH UMLS (Unified Medical Language System)

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