Graphics Reference
In-Depth Information
Boxplots
6.7
Assessing Three-way Interaction
6.7.1
Cochran and Cox (
) report on an experiment to assess the effect of electrical
stimulationtopreventtheatrophyofmuscletissueinrats.hisexperimentcontained
aresponsevariable
wt.d
(weightoftreatedmuscle),acovariate
wt.n
(weightofun-
treated muscle), and three factors,
current
,
n.treat
,and
minutes
.hereare
two replications of the entire experiment. It is desired to model
wt.d
as a function
of the other variables. he datafile is (hh/datasets/muscle.dat).
Figure
.
is designed to assess whether there is a significant three-way interac-
tion. he three-way interaction is not significant in this example. If there were a sig-
nificant three-wayinteraction, thepatterns inboxplotsinadjacent rowsandcolumns
would not be the same. For example, we note a hint of a difference in the
y.adj ~
minutes
behavior across panels. It has a negative slope in the
galvanic ~ 3
panel and a positive slope in the
faradic ~ 3
panel, but a positive slope in the
galvanic ~ 6
panelandanegative slopeinthe
faradic ~ 6
panel.However,
an ANOVA table for a full model containing the three-factor interaction indicates
Figure
.
.
hree-way interactions of all effects. One of the (
! =
) possible orderings (Files:
hh/dsgn/code/cc
.s, hh/csc/code/cc
-csc.s)