Chaughule, Thorat - Statistical Analysis/Design of Experiments
sponse. For example, in many hot air drying experiments both temperature and humidi-
ty affect a response, but the laboratory equipment can only control temperature; humid-
ity can be measured but is difficult to control. In such experiments temperature would
be regarded as an experimental factor and humidity as a covariate. A test run is a single
factor-level combination for which an observation (response) is obtained. Repeat tests
are two or more observations that are obtained for a specified combination of levels of
the factors. Repeat tests are conducted under as identical experimental conditions as
possible, but they need not be obtained in back-to-back test runs. Repeat tests should
not be two or more analytical determinations of the same response; they must be two or
more identical but distinct test runs.
Table 8.2. Experimental-Design TerminologyTerminology
DefinitionBlock Group of homogeneous experimental units.
Confounding One or more effects that cannot unambiguously be attri-
buted to a single factor or interaction.
Covariate An uncontrollable variable that influences the response
but is unaffected by any other experimental factors.
Design (layout). Complete specification of experimental test runs, includ-
ing blocking, randomization, repeat tests, replication, and
the assignment of factor–level combinations to experi-
mental units.
Effect Change in the average response between two factor–level
combinations or between two experimental conditions.
Experimental region (fac-
tor space)All possible factor–level combinations for which experi-
mentation is possible.
Factor A controllable experimental variable that is thought to
influence the response
Homogeneous experimen-
tal unitsUnits those are as uniform as possible on all characteris-
tics that could affect the response.
Interaction Existence of joint factor effects in which the effect of each
factor depends on the levels of the other factors.
Level Specific value of a factor
Noise The natural variations that occur in a process when all
conditions are maintained at the same level
Repeat tests Two or more observations that have the same levels for
all the factors
Replication. Repetition of an entire experiment or a portion of an ex-
periment under two or more sets of conditions.
Response Outcome or result of an experiment.
Test run Single combination of factor levels that yields an obser-
vation on the response.
Unit (item). Entity on which a measurement or an observation is
made; sometimes refers to the actual measurement or
observation
Replications are repetitions of a portion of the experiment (or the entire experiment)
under two or more different conditions, for example, on two or more different days. Ex-
perimental responses are only comparable when they result from observations taken on
homogeneous experimental units. Homogeneous experimental units do not differ from