I don't usually respond to anonymous querents, but the problem is
intriguing.
On Sat, 30 Sep 2000 [EMAIL PROTECTED] wrote:
> I constructed a D-optimal design for 6 continuous variables, each at
> three levels.
Is that 6 predictors, or 5 predictors and a response variable?
> I have 31 runs.
By this I understand you to mean that there are 31 observations in
the data set. If this is in error, perhaps you could describe things
a tad more explicitly.
> My initial model includes, all the main effects, all interactions and
> polynomial terms.
Then you must be running out of d.f. for error. You have only 30 d.f.
in total, and you've described a model requiring 31 d.f. for 5
predictors, even without considering the polynomial terms.
> I was only able to remove 3 of the terms using step wise regression.
> My final model has an R square of 0.9954, which looks very artificial.
A consequence of the variety required in the model vs. the variety
supplied in the data.
> My adjusted R square is also very close to R Square.
As is natural for R very close to 1.
> Does anyone have any suggestion on what could have gone wrong or if
> there is a different analysis technique that I can use?
You should be able to deal with it from here.
> I even did a lack of fit test, and the interaction terms and square
> terms were significant.
If as I suspect your error SS was virtually zero, this is to be expected.
One still wonders what you were using for error d.f. Perhaps your claim
of modelling "all interactions", which I interpreted as meaning "up to
and including 5-way interactions" (or 6-way interactions, if you meant
that your model began with 6 predictors), was hyperbolic?
-- DFB.
----------------------------------------------------------------------
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
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