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Sample Size Determination Example

AREA: Means | Neurology

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Teva Multiple Sclerosis Research

TitleThree times weekly glatiramer acetate in relapsing–remitting multiple sclerosis

Objective: To assess the efficacy and safety of glatiramer acetate (GA) 40mg administered 3× weekly (tiw) compared with placebo in patients with relapsing–remitting multiple sclerosis (RRMS).

Year: 2013

Source: Annals of Neurology

Link: https://onlinelibrary.wiley.com/doi/abs/10.1002/ana.23938

Clinical Area: Neurology

Sample Size Section in Paper/Protocol:

“A sample size of 1,350 patients (900 patients in the GA treatment group and 450 in the placebo group) was considered necessary to provide 90% power to detect a statistically significant difference in the total number of confirmed relapses between the treatment groups. The calculation accounted for an expected ARR of 0.35 in an untreated population, an expected ARR of ≤0.245 in the GA-treated population, and a dropout rate of 15%.”


Summary of Necessary Parameter Estimates for Sample Size Calculation:

 Parameter Value
 Significance Level (2-Sided)  0.05
 Control Rate 0.35
 Rate Ratio 0.7
 Dispersion Parameter Not Given
 Exposure Time 1
Sample Size Ratio 2
Power 90%
Expected Dropout 15%

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Step 1:
Select the Two Group Test Comparing Incidence Rates using the Negative Binomial Model table 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 10- Img 01- Two Group Test Comparing Incidence Rates using the Negative Binomial Model

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

A 15% dropout rate can be accounted for by a 15% reduction in average exposure time and thus setting exposure time to 0.85 in this example.

Sample Size Calculator Example- nQuery- Example 10- Img 2- Two Group Test Comparing Incidence Rates using the Negative Binomial Model

 

Step 3:
Click Run to solve for the sample size.

Sample Size Calculator Example- nQuery- Example 10- Img 03- Two Group Test Comparing Incidence Rates using the Negative Binomial Model

Step 4:
Once the calculation is completed, nQuery Advanced provides an output statement summarizing the results. It States:

Output Statement:
The comparison of two event (incidence) rates using a Negative Binomial model with a group 1 sample size of 440 and a group 2 sample size of 880 achieves 90.03% power at the 0.05 significance level when the mean incidence rate in group 1 is 0.35 and the rate ratio is 0.7. This assumes the mean exposure time was 0.85 and the two groups have a common dispersion parameter of 0. This assumes the variance under the null hypothesis was calculated using the (Control Group Rate/True Rates/Maximum Likelihood) method.

 

Sample Size Calculator Example- nQuery- Example 10- Img 04- Two Group Test Comparing Incidence Rates using the Negative Binomial Model

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