Chapter Contents 
Previous 
Next 
Details of the FACTEX Procedure 
See FACTEX9 in the SAS/QC Sample Library 
Factorial designs involving two levels are the most popular experimental designs. For twolevel designs, it is assumed that the response is close to linear over the range of the factor levels. To check for curvature and to obtain an independent estimate of error, you can replicate points at the center of a twolevel design. Adding center points to the design does not affect the estimates of factorial effects.
To construct a design with center points, you first create a data set with factorial points using the FACTEX procedure and then augment it with center points by using a simple DATA step. The following example illustrates this technique.
A researcher is studying the effect of three twolevel factors current (CURRENT), voltage (VOLTAGE), and time (TIME) by conducting an experiment using a complete factorial design. The researcher is interested in studying the overall curvature over the range of factor levels by adding four center points.
You can construct this design in two stages. First, create the basic 2^{3} design with the following statements:
proc factex; factors current voltage time; output out=factdat current nvals=(12 28 ) voltage nvals=(100 200) time nvals=(50 60 ); run;
Next, create the center points and append to the basic design as follows:
data center(drop=i); do i = 1 to 4; current = 20; voltage = 150; time = 55; output; end; data cpdesgn; set factdat center; run; proc print data=cpdesgn; run;
The design saved in the data set CPDESIGN is displayed in Output 15.3.1. Observations 1 to 8 are the factorial points, and observations 9 to 12 are the center points.
Output 15.3.1: A 2^{3} Design with Four Center Points

Chapter Contents 
Previous 
Next 
Top 
Copyright © 1999 by SAS Institute Inc., Cary, NC, USA. All rights reserved.