Epi
Treatment Effect
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Treatment Effect
See Also
Statistical Terms
(
Statistical Significance
,
P-Value
,
Confidence Interval
,
Clinical Significance
,
Statistical Heterogeneity
)
Approach
See
Contingency Grid
for description of setting up this grid
Given
We are studying whether an event is less likely to occur in an experimental group, then in a control group
Create a hypothetical grid
Label the grid top
Event (or E+)
No Event (or E-)
Total patients
Label the grid left
Control (or T-)
Experimental (or T+)
Total patients
Fill in the next row (Control Row)
Event = a
No Event = b
Total = a + b
Fill in the top row (Experimental Row)
Event = c
No Event = d
Total = c + d
Fill in the bottom row (Total row)
Event = a + c
No Event = b + d
Total = a + b + c + d
Calculations
Control event rate (CER)
Risk of event from control intervention = a / (a+b)
Experimental event rate (EER)
Risk of event from experimental intervention = c / (c+d)
Relative Risk
(RR)
RR = EER / CER
Hazard ratio (HR) is a similar concept related to survival
Relative Risk Reduction
(RRR)
RRR = 1 - RR
Absolute Risk Reduction
(ARR)
ARR = CER - EER
Number Needed to Treat
(NNT)
NNT = 1 / ARR
Example
Dabigatran
versus
Warfarin
in
CVA Prevention
See
Contingency Grid
for description of setting up this grid
Given
Example: Does CVA occur less often in
Atrial Fibrillation
with
Dabigatran
(experimental) than with
Warfarin
(control)
Connolly (2009) N Engl J Med 361:1139-51 [PubMed]
Create a grid
Label the grid top
CVA (or E+)
No CVA (or E-)
Total patients
Label the grid left
Warfarin
(or T-)
Dabigatran
150 mg (or T+)
Total patients
Fill in the next row (
Warfarin
Row)
CVA = a = 199
No CVA = b = 5823
Total = a + b = 6022
Fill in the top row (
Dabigatran
Row)
CVA = c = 134
No CVA = d = 5942
Total = c + d = 6076
Fill in the bottom row (Total row)
CVA = a + c = 333
No CVA = b + d = 11765
Total = a + b + c + d = 12098
Calculations
Warfarin
control event rate (CER)
Risk of CVA on
Warfarin
= a / (a+b) = 199/6022 = 0.033 (3.3%)
Dabigatran
experimental event rate (EER)
Risk of event from experimental intervention = c / (c+d) = 134/6076 = 0.022 (2.2%)
Relative Risk
(RR)
RR = EER / CER = 0.022 / 0.033 = 0.667
Relative Risk Reduction
(RRR)
RRR = 1 - RR = 1 - 0.667 = 0.333
Absolute Risk Reduction
(ARR)
ARR = CER - EER = 0.033 - 0.022 = 0.011
Number Needed to Treat
(NNT)
NNT = 1 / ARR = 1 / 0.011 = 90.9
References
Hersh (2014) Evidence Based Medicine, AMIA’s CIBRC Online Course
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