####
**II. 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

- Label the grid top
- 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

- Control event rate (CER)

####
**III. 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

- Label the grid top
- 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

- Warfarin control event rate (CER)

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**IV. References**

- Hersh (2014) Evidence Based Medicine, AMIA’s CIBRC Online Course