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Sample Size Determination Example

AREA: Survival Analysis | Cardiology

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Pfizer Atrial Fibrillation Research

TitleApixaban versus Warfarin in Patients with Atrial Fibrillation

Objective: In this randomized, double-blind trial, we compared apixaban (at a dose of 5mg twice daily) with warfarin (target international normalized ratio, 2.0 to 3.0) in 18,201 patients with atrial fibrillation and at least one additional risk factor for stroke. The primary outcome was ischemic or hemorrhagic stroke or systemic embolism. The trial was designed to test for noninferiority, with key secondary objectives of testing for superiority with respect to the primary outcome and to the rates of major bleeding and death from any cause.

Year: 2011

Source: New England Journal of Medicine

Link:  http://www.nejm.org/doi/full/10.1056/NEJMoa1107039

Clinical Area: Cardiology

Sample Size Section in Paper/Protocol:

''The primary non inferiority hypothesis required that apixaban preserve at least 50% of the relative reduction in the risk of stroke or systemic embolism associated with warfarin (62%) in six previous, major randomized, controlled trials.10 This hypothesis provided a lower 95% confidence interval of 1.88 for the relative risk with placebo as compared with warfarin, and one half of this value was 1.44 (or 1.38 on a log scale).
We estimated that with the occurrence of the primary outcome in 448 patients, the study would have 90% power to ensure that the upper boundary of the 99% confidence interval for the relative risk would be less than 1.44 and that the upper boundary of the 95% confidence interval for the relative risk would be less than 1.38, on the assumption that apixaban and warfarin had identical effects. On the basis of the overall event rate during the trial, we planned to recruit 18,000 patients in order to reach this number of events with approximately 2 years of follow-up.”

 

From Protocol: Incidence rate = 1.2 events per hundred person-years
Average Follow-up = 2.1 years  | Expected Dropout = 1%

Note: Only the 99% confidence interval with 1.44 relative risk analysis replicated since protocol states this was analysis with higher sample size.

Summary of Necessary Parameter Estimates for Sample Size Calculation:

 Parameter Value
 Significance Level (1-Sided) 0.005
 Non-Inferiority Margin 1.44
 Power 90%
 Incidence Rate per Year 0.012
 Average Follow-Up (Years) 2.1%
Expected Dropout 1%

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Step 1: 
Select the Non-inferiority test for Two Exponential Survival Curves from the Select Test Design & Goal window.

This can be done using the radio buttons or alternatively, you can use the search bar at the end of the Select Test Design & Goal window.

Sample Size Calculator Example- nQuery- Example 20- Img 01- Non-inferiority test for Two Exponential Survival Curves

Step 2:
Enter the parameter values for sample size calculation taken from the study protocol

Sample Size Calculator Example- nQuery- Example 20- Img 02- Non-inferiority test for Two Exponential Survival Curves

Step 3:
Once Power is entered, sample size is calculated automatically.

This gives a number of events of 448, as per the study protocol.

 

Sample Size Calculator Example- nQuery- Example 20- Img 03- Non-inferiority test for Two Exponential Survival Curves

Step 4:
Calculate the sample size based on the number of events, the incidence rate and the average follow up period.

Sample Size Calculator Example- nQuery- Example 20- Img 04- Non-inferiority test for Two Exponential Survival Curves

Adjust sample size for 1% dropout rate

Sample Size Calculator Example- nQuery- Example 20- Img 05- Non-inferiority test for Two Exponential Survival Curves

This gives approximately 18000 as per the protocol

 

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