Chapter 34: The GLMPOWER Procedure

Overview

Power and sample size analysis optimizes the resource usage and design of a study, improving chances of conclusive results with maximum efficiency. The GLMPOWER procedure performs prospective power analysis for linear models, with a variety of goals:

• determining the sample size required to get a significant result with adequate probability (power)

• characterizing the power of a study to detect a meaningful effect

• conducting what-if analyses to assess sensitivity of the power or required sample size to other factors

Here prospective indicates that the analysis pertains to planning for a future study. This is in contrast to retrospective analysis for a past study, which is not supported by this procedure.

The statistical analyses that are covered include Type III tests and contrasts of fixed class effects in univariate linear models, optionally with covariates. The covariates can be continuous or categorical. Tests and contrasts involving random effects are not supported. For power and sample size analyses in a variety of other statistical situations, see Chapter 57, The POWER Procedure.

Input for PROC GLMPOWER includes the components considered in study planning:

• design (including subject profiles and their allocation weights)

• statistical model

• contrasts of class effects

• significance level (alpha)

• surmised response means for subject profiles (often called cell means )

• surmised variability

• power

• sample size

In order to identify power or sample size as the result parameter, you designate it by a missing value in the input. The procedure calculates this result value over one or more scenarios of input values for all other components.

You specify the design and the cell means using an exemplary data set , a data set of artificial values constructed to represent the intended sampling design and the surmised response means in the underlying population. You specify the model and contrasts using MODEL and CONTRAST statements similar to those in the GLM, ANOVA, and MIXED procedures. You specify the remaining parameters with the POWER statement, which is similar to analysis statements in the POWER procedure.

In addition to tabular results, PROC GLMPOWER produces graphs. You can produce the most common types of plots easily with default settings and use a variety of options for more customized graphics. For example, you can control the choice of axis variables , axis ranges, number of plotted points, mapping of graphical features (such as color , line style, symbol, and panel) to analysis parameters, and legend appearance.

The GLMPOWER procedure is one of several tools available in SAS/STAT software for power and sample size analysis. PROC POWER covers a variety of more basic analyses such as t tests, equivalence tests, confidence intervals, binomial proportions , multiple regression, and one-way ANOVA. The Power and Sample Size application provides a user interface and implements many of the analyses supported in the procedures.

The following sections of this chapter describe how to use PROC GLMPOWER and discuss the underlying statistical methodology. The Getting Started section on page 1930 introduces PROC GLMPOWER with examples of power computation for a two-way analysis of variance. The Syntax section on page 1935 describes the syntax of the procedure. The Details section on page 1945 summarizes the methods employed by PROC GLMPOWER and provides details on several special topics. The Examples section on page 1951 illustrates the use of the GLMPOWER procedure with several applications.

For more discussion and examples on power and sample size analysis for linear models, refer to Castelloe and O Brien (2001), O Brien and Shieh (1992), Muller et al. (1992), and O Brien and Muller (1993). For additional discussion on general power and sample size concepts, refer to Castelloe (2000), Muller and Benignus (1992), and Lenth (2001).