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 non-treatment) 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.