1. HelpDesk Answers Synthesizing the Evidence
Corey Lyon, D.O.
University of colorado

2. Objectives
Gain an understanding of how to use statistics in your manuscript
Define common statistical term used in evidence based medicine
Exposure to the 2X2 table in converting data to user friendly data
Learn how to use online tools to assist in evidence synthesis

3. Statistical Evidence Cause the most fear among faculty new to writing
You do not need to learn how to DO the statistics
You just need to understand the statistics
Keep it simple 3

4. 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. 4

5. EBM Glossary 95 percent confidence interval Odds Ratio
An estimate of certainty. It is 95% certain that the true value lies within the given range. A narrow CI is good. A CI that spans 1.0 calls into question the validity of the result.
Odds Ratio
Compares whether the probability of a certain event is the same for two groups.
An odds ratio of 1 implies that the event is equally likely in both groups. OR >1 implies that the event is more likely in the first group. OR <1 implies that the event is less likely in the first group. 5

6. EBM Glossary (cont.) Relative Risk (RR)
direct measure comparing the probabilities in two group
also known as the risk ratio
RR equal to 1 implies that the event is equally probable in both groups. A RR >1 implies that the event is more likely in the first group. A RR <1 implies that the event is less likely in the first group
Example – Risk of vomiting with Zofran vs Placebo in pediatric pts in the ER
RR 0.45, 95% CI
Since the RR is less than 0, and the CI does not cross 1
This means the risk of vomiting in the Zofran group is
less compared to placebo 6

7. EBM Glossary (cont.) Number needed to treat – NNT
The number of patients who need to receive an intervention instead of the alternative in order for one additional patient to benefit.
Number needed to harm – NNH
The number of patients who need to receive an intervention instead of the alternative in order for one additional patient to experience an adverse event. 7

8. EBM Glossary - Diagnosis
Sensitivity – SnOut
the proportion of truly diseased persons who are identified as diseased by the test
In other words, a sensitive test has very few false negatives; SnOut (Sensitivity rules Out dz)
Specificity - SpIn
the proportion of truly non-diseased persons who are identified as being non-diseased by the test; SpIn (Specificity rules In disease)
In other words, a specific test has very few false positives 8

9. EBM Glossary – Diagnosis (cont.)
Likelihood ratio LR
LR >1 indicates an increased likelihood of disease, LR <1 indicates a decreased likelihood of disease. The most helpful tests generally have a ratio of less than 0.2 or greater than 5 9

10. Confidence Intervals Provides a measure of the study results
Places a clear quantification of the effect
Contrast to a P value – arise from significance testing
Tells nothing of the size or direction of the difference
CI’s reveal the treatment benefit/harm and strength of evidence
CI’s – “a range of values within which we
can be 95% sure that the true values lie” 10

11. Example: Confidence Ratio11

12. Confidence Interval (cont.)
A 95 percent confidence interval indicates that if the study were repeated 100 times, the study results would fall within this interval 95 times.
For example, if a study found that a test was 80 percent specific with a 95 percent confidence interval of 74 to 85 percent, the specificity would fall between 74 and 85 percent 95 times if the study were repeated 100 times. 12

13. Example of Confidence Intervals
Can measure multiple clinical measures of interest
Odds Ratio- ER use for asthma exacerbation in prednisone group vs placebo;
OR 0.54, 95% CI
Since the OR is less than 0, and the CI does not cross 1, the odsd of going to the ER in the prednisone group is less compared to placebo
NNT – ER use for asthma; Prednisone vs placebo;
NNT 7.1, 95% CI – The actual NNT is somewhere between 4 and 30
Hazard Ratio – ACS in pts taking Plavix and PPI
HR 1.35, 95% CI
Since the HR is greater than 1, and the CI does not cross 1, the odds of have a MI in the Plavix and PPI group is greater than the Plavix alone
Specificity – GP in diagnosing depression
81.3%, 95% CI 74.5 – 87.3
RR – Return to Care in pts receiving Zofran vs placebo
RR 1.34, 95% CI 0.77 – 2.35 – CI crosses 1; nonsignificant 13

14. 2X2 Table for Outcome of Interest
User-Friendly Stats
2X2 Table for Outcome of Interest
Experimental Treatment
Placebo/ Comparison + - a b 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
Number Needed to Treat: NNT = 1 / ARR 14

15. Example: User-Friendly Stats15

16. Example: User-Friendly Stats
Primary End Points (first major cardiovascular events) + -
142
8759
251
8650
Experimental treatment
Placebo/
Comparison
Control Event Rate: Cer = c / c + d = 251/ = 0.028
Experimental Event Rate: Eer = a / a + b = 142/ = 0.016
Absolute Risk Reduction: ARR = Cer – Eer = – = 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 16

18. Example: User-Friendly Stats Non – Fatal Myocardial Infarction22
8879 62
8839
Experimental
treatment
Placebo/
Comparison
Control Event Rate: Cer = c / c + d = 62/ =
Experimental Event Rate: Eer = a / a + b = 22/ =
Absolute Risk Reduction: ARR = Cer – Eer = – =
Relative Risk Reduction: RRR = / = 0.65
Number Needed to Treat: NNT = 1 / = 223 18

19. Summary Statistics are the heart of the evidence for Help Desk Answers
Convert the data into friendlier terms
Confidence Intervals
NNT
+/- LR
Use the 2X2 table to convert data to user friendly terms