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.