1

 Critically Appraising the Evidence:
 Statistics for Harm and Etiology

2

 Clinical Statistics Calculator (Excel)
 Statistics for:
 Harm/Etiology & Prognosis
 Control Event Rate (CER) & Experimental Event Rate (EER)
 Number Needed to Harm (NNH)
 Absolute Risk Increase (ARI)
 Relative Risk (RR)
 Odds
 Odds Ratio (OR)
 Practice Exercises

3

 If available, find the best evidence in secondary sources where analysis
has already occurred.
 If not preassessed, use critical appraisal worksheets to help you
through the process.

4

 Understanding the Limitations of the Author’s Analyses and
Interpretations of the Data
 Assessing Internal Validity
 Assessing External Validity
 Identifying Potential Confounding Variables

5

 View movie as:
 QuickTime (.mov)
 Flash (.swf)
 Doubleclick on video for fullscreen mode.

6


7

 Experimental Event Rate (EER)
 The proportion of patients (in the intervention) who experienced the
target outcome
 Control Event Rate (CER)
 The proportion of patients (in the comparison group) who experienced
the target outcome

8

 Experimental Event Rate (EER)
 Control Event Rate (CER)

9

 The estimated number of people treated for each patient the treatment
harms beyond baseline/control risk.
 So smaller numbers indicate greater probability of harm as a result of
the treatment

10

 The calculation of NNH is the same as that of NNT, but the
interpretation is differentiated by the adverse outcomes present in harm
studies. Here the term NNH effectively replaces NNT.
 Similarly Absolute Risk Increase (ARI) is calculated in the same way
that ARR is calculated but the intervention increases the amount of harm
instead of therapeutic gain.
 NNH = 1/ARI = 1/CER – EER

11


12

 The number of treated/exposed patients with the target outcome for every
patients in the control with the target outcome
 (Also used in therapy articles)
 RR = EER / CER

= (a/(a+b)) / (c/(c+d))

13


14

 The number of times the target outcome occurred in patients exposed to
the risk for each time the target outcome occurred in patients not
exposed to the risk.

15

 OR = (a/b) / (c/d)
 =
a*d / b*c
 A measure of association
 When large, there is greater association

16

 Critical Appraisal Practice Exercises

17

 EBM Glossary
 Critical Appraisal Practice Exercises

18

