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RE: st: Ordplot interpretation


From   "Nick Cox" <[email protected]>
To   <[email protected]>
Subject   RE: st: Ordplot interpretation
Date   Mon, 26 Jul 2004 11:05:12 +0100

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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