HELPDESK ANSWERS
SYNTHESIZING THE EVIDENCE:
Statistical Evidence
OVERVIEW
 Learn which statistics are most helpful in relaying
clinical information.
 Discover helpful tools in working with statistics.
 Convert study results into user-friendly statistics.
PUTTING STATISTICS INTO ENGLISH
Fictitious Case Example
A now unethical RCT of sunscreen vs. placebo for melanoma shows a rate of 1/1000 = .001 for 20-year sunscreen
use vs. 5/1000 = .005 for placebo in the prevention of melanoma.
EER (Experimental Event Rate) = .001
“The proportion of those who developed melanoma over 20 years in the sunscreen group was 1 out of 1000.”
ARR (Absolute Risk Reduction) = EER-CER = .004
“For every 1000 patients using sunscreen, 4 cases of melanoma are averted.”
NNT (Number-needed-to-treat) = 1/ARR = 250
“250 patients would need to be treated with sunscreen rather than placebo for 20 years to prevent one additional
case of melanoma.”
RR (Relative Risk) = EER/CER = .20
“People who use sunscreens for 20 years have less than a fifth of the risk of developing melanoma of those who
use a placebo. Alternatively, people who do not use sunscreens have a 5 times greater risk of developing
melanoma over 20 years as compared to those who use sunscreens.”
Risk remaining: “Just 20% of the original risk of developing melanoma remains for those who use
sunscreen.”
RRR (Relative Risk Reduction) = (EER-CER)/CER or 1 – RR = .80
“Sunscreen use decreases the risk of developing melanoma by 80% compared with no sunscreen.”
Risk removed: “80% of the risk of developing melanoma is removed by using sunscreen.”
USER-FRIENDLY STATISTICS
 We’ll focus on a few key statistics in the HelpDesk Answer Project
 Confidence Interval (CI) Quantifies the uncertainty in measurement. It is
usually reported as a "(95% CI 5-15) which is the range of values within which
we can be 95% sure that the true value for the whole population lies. For
example, for an NNT of 10 with a 95% CI of 5 to 15, we would have 95%
confidence that the true NNT value lies between 5 and 15.
 Likelihood ratio The likelihood that a given test result would be expected in
a patient with the target disorder compared with the likelihood that this same
result would be expected in a patient without the target disorder.
 Number needed to treat (NNT) The number of patients that we need to
treat with a specified therapy in order to prevent one additional bad outcome.
Or the NNT is the number of patients who must be treated for one patient to
benefit. Calculated as the inverse of the absolute risk reduction (1/ARR).
STATISTICAL EVIDENCE
 Convert study results into user-friendly statistics:
 Confidence Intervals
 Likelihood Ratios
 Number Needed to Treat/Harm
 Use tables to summarize or compare studies.
 Statistics ARE the ‘heart’ of the evidence. If the
author is unsure, the reader will be unsure. If in
doubt, seek experts.
User-Friendly Stats
2X2 Table for Outcome of Interest
+
-
Experimental
Treatment
a
b
Placebo/
Comparison
c
d
a. Control Event Rate: Cer = c / c + d
b. Experimental Event Rate: Eer = a / a + b
c. Absolute Risk Reduction: ARR = Cer – Eer
d. Relative Risk Reduction: RRR = ARR / Cer
e. Relative Risk Ratio = Eer / Cer
f. Number Needed to Treat: NNT = 1 / ARR
http://www.cebm.net/index.aspx?o=1160
Example: User-Friendly Stats
Example: User-Friendly Stats
Primary End Points
(first major cardiovascular events)
+
-
Experimental
treatment
142
8759
Placebo/
Comparison
251
8650
Control Event Rate: Cer = c / c + d = 251/251 + 8650 = 0.028
Experimental Event Rate: Eer = a / a + b = 142/142 + 8759 = 0.016
Absolute Risk Reduction: ARR = Cer – Eer = 0.028 – 0.016 = 0.012
Relative Risk Reduction: RRR = ARR / Cer = 0.012/0.028 = 0.43
Number Needed to Treat: NNT = 1 / ARR = 1/0.012 = 83
http://www.cebm.net/index.aspx?o=1160
Example: User-Friendly Stats
Non – Fatal Myocardial Infarction
Experimental
treatment
22
8879
Placebo/
Comparison
62
8839
Control Event Rate: Cer = c / c + d = 62/62 + 8839 = 0.00696
Experimental Event Rate: Eer = a / a + b = 22/22 + 8879 = 0.00247
Absolute Risk Reduction: ARR = Cer – Eer = 0.00696 – 0.00247 = 0.00449
Relative Risk Reduction: RRR = 0.00449/0.00696 = 0.65
Number Needed to Treat: NNT = 1 / 0.00449 = 223
http://www.cebm.net/index.aspx?o=1160
USER-FRIENDLY STATISTICS
 Review the example given until you feel comfortable in
following the numbers used and calculations made.
 Additional practice examples are available in the quiz
portion of this module.
 Stats calculators are available throughout the web and
links are available through the module home page