User's Guides to the Medical Literature
Evidence-Based Medicine Working Group

Are the Results Valid?
Was the assignment of patients to treatment randomized?
Were all patients who entered the trial accounted for and attributed at its conclusion?
Were patients, clinicians and study personnel kept "blind" to treatment received?
Were the groups similar at the start of the trial?
Aside from the experimental intervention were the groups treated equally?

What Are the Results?
How large is the treatment effect?
How precise is the estimate of the treatment effect?

Will the Results Help Me in My Patient Care?
Can the results be applied to my patients?
Were all clinically relevant outcomes considered?
Are the benefits worth the harms and costs?

JAMA 1993; 270: 2598-2601
JAMA 1994; 271: 59-63

Estimating the size of the Treatment Effect
Outcome +Outcome -
Treated (Y)ab
Control (X)cd

Risk of Outcome
Y = a/(a+b)
X = c/(c+d)

Relative Risk, or Risk Ratio, is the ratio of risk in treated group (Y) to the risk in the control group (X). RR =Y/X

Relative Risk Reduction is the percent reduction in risk in treated group (Y) compared to controls (X):
RRR = (1 - RR) = (1 -Y/X) x 100%
RRR = [(X - Y)/X] x 100%

Absolute Risk Reduction is the difference in risk between control group (X) and the treated group (Y): ARR = X - Y

Number Needed to Treat is the inverse of the ARR: NNT = 1/ARR = 1/(X - Y)

"An NNT of 15, means that 15 patients must be treated over a given period of time to prevent one adverse outcome"

Finding Articles About Therapy
Clinical Trial (pt)
Randomized Controlled Trial (pt)

[see ACPJ Club 1994 Jul-Aug: A10-A12]