pg_0048
5.
Process Improvement
5.3.
Choosing an experimental design
5.3.3.
How do you select an experimental design.
5.3.3.2.
Randomized block designs
Blocking to
"remove" the
effect of
nuisance
factors
For randomized block designs, there is one factor or variable that is of
primary interest. However, there are also several other nuisance
factors.
Nuisance factors are those that may affect the measured result, but are
not of primary interest. For example, in applying a treatment, nuisance
factors might be the specific operator who prepared the treatment, the
time of day the experiment was run, and the room temperature. All
experiments have nuisance factors. The experimenter will typically
need to spend some time deciding which nuisance factors are
important enough to keep track of or control, if possible, during the
experiment.
Blocking used
for nuisance
factors that
can be
controlled
When we can control nuisance factors, an important technique known
as blocking can be used to reduce or eliminate the contribution to
experimental error contributed by nuisance factors. The basic concept
is to create homogeneous blocks in which the nuisance factors are held
constant and the factor of interest is allowed to vary. Within blocks, it
is possible to assess the effect of different levels of the factor of
interest without having to worry about variations due to changes of the
block factors, which are accounted for in the analysis.
Definition of
blocking
factors
A nuisance factor is used as a blocking factor if every level of the
primary factor occurs the same number of times with each level of the
nuisance factor. The analysis of the experiment will focus on the
effect of varying levels of the primary factor within each block of the
experiment.
Block for a
few of the
most
important
nuisance
factors
The general rule is:
"Block what you can, randomize what you cannot."
Blocking is used to remove the effects of a few of the most important
nuisance variables. Randomization is then used to reduce the
contaminating effects of the remaining nuisance variables.
5.3.3.2. Randomized block designs
http://www.itl.nist.gov/div898/handbook/pri/section3/pri332.htm (1 of 4) [5/7/2002 4:01:46 PM]
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