# Glossary of Terms for Evidence-Based Orthopaedics*

Journal of Bone & Joint Surgery - American Volume:
Evidence-Based Orthopaedics

The glossary below provides definitions for several terms found in the structured abstracts of the Evidence-Based Orthopaedics section, many of which may not be familiar to orthopaedic surgeons.

##### The following terms are commonly used in abstracts describing trials of therapeutic interventions.

EER is the experimental (new treatment) event rate.

CER is the control (old treatment or nontreatment) event rate.

CI (confidence interval) quantifies the uncertainty in measurement. CI is usually reported as a 95% CI, which is the range of values within which we can be 95% sure that the true value for the whole population lies.

##### When the experimental treatment reduces the risk for a bad event:

ARR (absolute risk reduction) is the absolute arithmetic difference in bad event rates between the experimental and control groups, calculated as |EER - CER|.

RRR (relative risk reduction) is the proportional reduction in bad event rates between the experimental and control groups, calculated as |EER - CER|/CER and accompanied by a 95% CI.

##### When trying to interpret the benefit of a new therapy, the NNT can be helpful to clinicians.

NNT (number needed to treat) is the number of patients who need to be treated to prevent 1 additional bad outcome, calculated as 1/ARR, rounded up to the nearest whole number, and accompanied by a 95% CI.

When the experimental treatment increases the probability of a good event:

ABI (absolute benefit increase) is the absolute arithmetic difference in good event rates between the experimental and control groups, calculated as |EER -CER|.

RBI (relative benefit increase) is the proportional increase in good event rates between the experimental and control groups, calculated as |EER - CER|/CER and accompanied by a 95% CI.

NNT (number needed to treat) is the number of patients who need to be treated to achieve 1 additional favorable outcome, calculated as 1/ARR, rounded up to the nearest whole number, and accompanied by a 95% CI.

##### When the experimental treatment increases the probability of a bad event:

ARI (absolute risk increase) is the absolute arithmetic difference in bad event rates between groups, when the experimental treatment harms more patients than the control treatment, calculated as |EER - CER|.

RRI (relative risk increase) is the proportional increase in bad event rates between the experimental and control groups, calculated as |EER - CER|/CER and accompanied by a 95% CI.

NNH (number needed to harm) is the number of patients that, if they received the experimental treatment, would lead to 1 additional person being harmed compared with the number of patients who received the control treatment, calculated as 1/ARI, rounded up to the nearest whole number, and accompanied by a 95% CI.

*Based on information from the American College of Physicians Journal Club.

Copyright 2001 by The Journal of Bone and Joint Surgery, Incorporated