Evidence-Based Medicine

GLOSSARY OF TERMS
IN EVIDENCE-BASED MEDICINE
This glossary in intended to provide explanation and guidance as to the
meanings of EBM term (a simple definition).
A
Absolute risk (AR)
The probability that an individual will experience the specified outcome during
a specified period. It lies in the range 0 to 1, or is expressed as a percentage.
In contrast to common usage, the word “risk” may refer to adverse events (such
as myocardial infarction) or desirable events (such as cure).
Absolute risk increase (ARI)
The absolute difference in risk between the experimental and control groups
in a trial. It is used when the risk in the experimental group exceeds the risk in
the control group, and is calculated by subtracting the AR in the control group
from the AR in the experimental group. This figure does not give any idea of
the proportional increase between the two groups: for this, relative risk (RR) is
needed.
Absolute risk reduction (ARR)
The absolute difference in risk between the experimental and control groups
in a trial. It is used when the risk in the control group exceeds the risk in the
experimental group, and is calculated by subtracting the AR in the experimental
group from the AR in the control group. The arithmetic difference in risk or
outcomes between treatment and control groups,. Example: if mortality is 30
percent in controls and 20 percent with treatment, ARR is 30-20=10 percent.
This figure does not give any idea of the proportional reduction between the
two groups: for this, relative risk (RR) is needed.
Allocation concealment
A method used to prevent selection bias by concealing the allocation sequence
from those assigning participants to intervention groups. Allocation concealment
prevents researchers from (unconsciously or otherwise) influencing which
intervention group each participant is assigned to.
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