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FAST STATS
A 5-minute Review for the Evidence-Based Practitioner
When evaluating medical literature,
there are two important concepts
absolute risk vs. relative risk
number needed to treat
Absolute Risk & Relative Risk
Relative Risk (RR)
compares the probability
of an event occurring in
the exposed group vs.
the non-exposed
group.
Absolute Risk (AR)
describes the incidence
of a condition in a
population.
Absolute Risk & Relative Risk
Example:
Drug A prevents heart attacks (MIs) but also increases the
risk of lung cancer. A placebo-controlled trial in 200
people (100 in each arm) found that there were three
heart attacks in the treated group and six heart attacks in
the placebo group. There were three cases of lung cancer
in the treated group, and one in the placebo group.
MIs
Lung Ca
Drug A
Placebo
3
3
6
1
Absolute Risk & Relative Risk
We might say that Drug A
reduces heart attack risk by
50% or cuts heart attack rate
in half.
We could also say that the
heart attack risk is reduced
by 3%.
Absolute Risk & Relative Risk
We could say that the
lung cancer risk
increases by 200%.
We could also say that
lung cancer risk
increases by 2%.
MIs
Lung Ca
Drug A
Placebo
3
3
6
1
Absolute Risk & Relative Risk
Absolute risk makes risks or benefits
look smaller.
Relative risk makes risks or benefits look
bigger.
Absolute Risk & Relative Risk
To be fair, both harms and benefits
should be presented in absolute
terms or relative terms.
Number Needed to Treat
Number Needed to Treat (NNT) is an important
means of evaluating data.
NNT is calculated by taking the reciprocal of
the absolute difference between
experimental groups.
Another way is by dividing 100 by absolute
risk.
Number Needed to Treat
3% of the treatment group and 6% of the control group
had heart attacks. The absolute difference is 3% because
6%-3%= 3% = 0.03
-ORThat means that 33 people must be treated with this drug
to prevent one heart attack.
Number Needed to Harm
Number Needed to Harm (NNH)
is also calculated by taking the reciprocal of the
absolute difference between experimental
groups.
Another way is by dividing 100 by absolute risk.
Number Needed to Harm
3% of the treatment group developed lung cancer,
compared to 1% of the placebo group. The absolute
difference is 2% or 0.02.
-ORSo the NNH is 50, meaning that we would expect one
additional case of lung cancer for every 50 people treated
with Drug A.
NNT vs. NNH
One out of every 33 people treated with Drug
A will be spared a heart attack; however, one
out of every 50 people will develop lung cancer
caused by Drug A.
MIs
Lung Ca
Drug A
Placebo
3
3
6
1
Congratulations!
Now you understand the difference
between absolute and relative risk, and
you can calculate NNT and NNH.
This information will help you make
evidence-based decisions
for your practice.
PharmedOut is a Georgetown University Medical Center
project that advances evidence-based prescribing, and
educates healthcare professionals about pharmaceutical
marketing practices. Learn more about us at
http://pharmedout.org/aboutus.htm.
Cover art by Andrea Sikora, inspired by Wojtek Kozak
Presentation layout by Andrea Sikora and Grace Doan
Nov 2011 version 2.4
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