|Details of the FACTEX Procedure
- BLOCKS block-specification ;
You use the BLOCKS statement to specify the number of blocks in the design or the
size of each block in the design. By default, the FACTEX procedure
constructs designs that do not contain blocks. If you use the
BLOCKS statement, you also need to use the MODEL statement or SIZE statement.
In particular, if you use the BLOCKS statement and your design is a fractional
factorial design, you must use the MODEL statement.
The two simplest explicit block-specifications that you can use are
- NBLOCKS=b, which specifies the number of blocks (b)
in the design
- SIZE=k, which specifies the number of runs (k) in each block
Use only one of these two options.
In all, there are six mutually exclusive block-specifications that
you can use, as described by the following list:
specifies the number of block pseudo-factors for
the design. The design contains a different block for each possible
combination of the levels of the block pseudo-factors. Values of
s are the integers 1, 2, and so on. See "Block Size Restrictions"
If each factor in the design has q levels, then
NBLKFACS=s specifies a design with
qs blocks. The size of
each block depends on the number of runs in the design, as
specified in the SIZE statement. If the design has n runs,
then each block has n/qs runs.
The following statement illustrates how to request
a two-level factorial design arranged in eight (23) blocks:
For more on pseudo-factors, see "Types of Factors"
specifies the number of blocks in the design. The values of
b must be a power of q, the number of levels of each factor
in the design.
See "Block Size Restrictions"
The size of each block depends on the number of runs in the
design, as specified in the SIZE statement. If the design has
n runs, then each block has n/b runs.
See "Example of a Full Factorial Design in Two Blocks"
for an illustration of this option.
The following statement illustrates how to specify a design arranged
in four blocks:
specifies the number of runs per block in the design.
The value k must be a power of q, the number of levels for
each factor in the design. The number of blocks
depends on the number of runs in the design, as specified in the
If the design has n runs, then it has
CAUTION: Do not confuse the SIZE= option in the BLOCKS statement with the
SIZE statement, which you use to specify the overall size of the design.
See "SIZE Statement"
for details of the SIZE statement.
The following statement illustrates how to specify blocks of size two:
constructs a blocked design with
the minimum number of runs per block, given all the other
characteristics of the design. In other words, the block
size is optimized. You cannot
specify this option if you specify any of the design size optimization
options in the SIZE statement
Equivalence of Specifications
The three explicit block-specifications are related to
each other, as demonstrated by the following example.
Suppose you want to
construct a design
for 11 two-level factors in 128
runs in blocks of size 8.
Since 128/24=128/16=8, three equivalent block specifications
The number of blocks and the number of runs in each block must be less than
the total number of runs in the design. Hence, there are some
restrictions on the block size.
Copyright © 1999 by SAS Institute Inc., Cary, NC, USA. All rights reserved.