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Step 1:
Search for the Two Sample Student’s t-test (equal variances) Unequal n’s 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.
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Step 2:
Enter the parameter values for sample size calculation taken from the study protocol.
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Step 3:
Once Power is entered, sample size is calculated automatically.
| This gives the sample sizes of 125 and 250 as per the protocol |
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Note: In the case of ANCOVA analysis, the only additional information necessary would be an estimate of R2 between the response and the baseline covariate (or average R2 if other covariates were used).
For example, using the ANCOVA sample size table available in nQuery for the above study and assuming a 10% R2 between baseline and the response would have yielded an adjusted total sample size of 336.
Step 4:
Once the calculation is completed, nQuery Advanced provides an output statement summarizing the results. It States:
| Output Statement: |
|
A two group t-test with a 5% two-sided significance level will have 90% power to detect the difference between a Group 1 mean, µ₁, of 363 and a Group 2 mean, µ₂, of 338 a difference in means of 25 assuming that the common standard deviation is 70, when the sample sizes in the two groups are 124 and 250, respectively (a total sample size of 374). |
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