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

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

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

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

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

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 0.33-0.62 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

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

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

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

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

Example: Confidence Ratio 11

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

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 0.34-0.86 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 4.0-30.3 – The actual NNT is somewhere between 4 and 30 Hazard Ratio – ACS in pts taking Plavix and PPI HR 1.35, 95% CI 1.18-1.56 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

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 http://ktclearinghouse.ca/cebm/practise/ca/calculators/statscalc 14

Example: User-Friendly Stats 15

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 = 251/251 + 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://ktclearinghouse.ca/cebm/practise/ca/calculators/statscalc 16

Example: User-Friendly Stats Non – Fatal Myocardial Infarction 22 8879 62 8839 Experimental treatment Placebo/ Comparison 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://ktclearinghouse.ca/cebm/practise/ca/calculators/statscalc 18

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