Joseph's interpretation is the same as mine.
The interaction is expressed as non-additivity
of effects, as expressed on the particular scale
used to show cumulatives. It is not necessary
that lines cross!
Incidentally, -ordplot- is stuck at Stata 7.
All of the features considered important
by the author have been implemented in
the same author's -distplot-, which
is downloadable from SJ 3(4) files or
from SSC.
The use of such plots for ordered
categorical variables is also discussed in
"Speaking Stata: Graphing categorical and compositional data"
SJ 4(2):190--215 (2004)
Nick
[email protected]
Joseph Coveney
>
> The interaction is evident in the greater rightward shift of
> the old Catholics
> from the young Catholics than of the old non-Catholics from
> the young non-
> Catholics.
>
> Don't expect these lines to cross as evidence of an
> interaction. A difference
> in the slopes of the lines of indicates heteroscedasticity
> and not necessarily
> interaction.
>
> These plots are analogous to dose-response function plots in
> pharmacology: to
> interpret them, draw a horizontal line (parallel to the x
> axis) through all of
> the plotted lines. Choose a value that allows the horizontal
> line to intersect
> all of the plotted lines; in this example -yline(0.5)- works.
> The distance
> along the horizontal line between points of intersection is a
> measure of
> proclivity to attend. Difference in the lengths along this
> line, i.e.,
> difference in this proclivity as a function of age, is
> greater among Catholics--
>
> > I have been using ordplot and thought that I understood it
> until I tried
> > the
> > Knoke and Burke example in the help file which states that
> the resultant
> > plot shows evidence of an interaction. I have been looking
> at this plot
> > for
> > a couple of days and I cannot see the interaction. I
> realise that I am
> > missing something blindingly obvious.
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