Calculating Sample Size for proportions with more than 2 groups

Test for linear trend in G x 2 table (logistic model)(PGT1)

nQuery Advisor provides sample size computations for a logistic model when the response variable is yes/no and the covariate is a discrete ordered variable such as fixed dose levels. Suppose the investigator wants to demonstrate a dose response to 5 doses of a new drug to prevent transplant rejection. The response variable is survival to a fixed time post transplant.

To bring up the PGT1 sample size table:

• Click on the File menu New option. The Study Goal and Design dialog box will appear.
• Select Proportions, >Two, Test.
• Select the second analysis method from the list of analysis methods.
• Click OK. The Trend across proportions, logistic model table will appear with no values entered. The following table shows the table with values for significance level, 2-sided test, and 5 groups entered.

To obtain a side table to allow you to enter the proportions expected to survive at each dose level, select Compute Effect Size from the Assistants menu or click on the δ button.

Enter the proportions expected at each dose level. The coefficients necessary to test for linear trend for equally spaced doses have already been entered in the table. If your doses are unequally spaced, replace these with the appropriate set of coefficients.

When all values have been entered, click on Compute to see the summary values shown.

After computing, click on Transfer to transfer these values back to the main table. For this study, the investigator wants to have 80% power. At least 19 animals would be required per dose group.

Note that the side table can be printed to Clipboard for inclusion in other documents, printed directly, and/or saved with the main table.