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