A quibble, and a question (or maybe several, of each), Rich:
On Thu, 25 Jan 2001, Rich Ulrich wrote, inter alia:
> By the way, if you have Pre-Post on one measure, you
> almost need to plot the points on a well-labeled graph
> (what is max, what is min?) before you BEGIN to draw
> conclusions.
> - Then, a *disordinal* interaction ...
Well, ALL interactions are disordinal, if by "interaction" one means
(e.g., in a 2-way ANOVA context)
cell mean - row mean - column mean + grand mean
(or equivalent, in ANCOV or regression contexts). What most people who
use "ordinal" and "disordinal" seem to mean is a plot of the cell means
(or of regression lines), with no adjustment for main effects: so, a
display that includes the interaction AND the main effects. I take it
that's what you mean here. Then: a disordinal display -- of what plot?
(As remarked in a thread a year or two ago, an interaction (displayed as
a plot of cell means or of regression lines) may appear ordinal from one
direction and disordinal from the other.)
> ... is just about the only EFFECT that I can think of, ...
It follows from the above that the "disordinal interaction" is NOT an
effect in what I take to be the usual sense (being, instead, a composite
of several effects).
> ... which cannot be discounted as artifactual or pretty trivial.
I do not follow your logic here. Could you explain it in more detail?
> If you don't have a control group on hand, you need to have
> information about what a control group SHOULD look like.
I have no argument with this point of view.
> For instance, in Education:
> If you group the highest IQ versus lowest,
> the "regression" for a year or two will be opposite:
> the highest will learn more new stuff, faster, and get further ahead.
Ah. Now this must surely yield an *ordinal* plot, which implies that
you would discount this phenomenon as "artifactual" or "pretty trivial"
(or perhaps both). Do I understand you correctly? Then, which; and
what (if applicable) is the artifact?
-- Don.
----------------------------------------------------------------------
Donald F. Burrill [EMAIL PROTECTED]
348 Hyde Hall, Plymouth State College, [EMAIL PROTECTED]
MSC #29, Plymouth, NH 03264 (603) 535-2597
Department of Mathematics, Boston University [EMAIL PROTECTED]
111 Cummington Street, room 261, Boston, MA 02215 (617) 353-5288
184 Nashua Road, Bedford, NH 03110 (603) 471-7128
=================================================================
Instructions for joining and leaving this list and remarks about
the problem of INAPPROPRIATE MESSAGES are available at
http://jse.stat.ncsu.edu/
=================================================================
