[R] Problem when fitting a constant response

[ZABALZA Isabelle]

    Hello,

I would like to treat a very simple case : to fit a linear model with 5
parameters (including main terms, interactions and quadratic terms using
a central composite design with 27 runs) for a constant response (e.g
resp = 100.0). The fitting process works and return me a good intercept
value (the value of my constant) and some negligeable effects (around
e-15). But, I don't understand some results :
the t values (or the F stat) indicates some influent parameters
(although they are negligeable in comparison with the intercept) and the
R2 value is not equal to 1 (Multiple r2 = 0.72, and adj r2 = -0.187),
although the residuals are near zero. I know, it is a very special case
(regression is not necessary), but it is just to understand what is
happening.

Thank you in advance,

Isabelle Zabalza-Mezghani

--
Isabelle Zabalza-Mezghani          Tel : 01 47 52 61 99
Institut Français du Pétrole       E-mail : isabelle.zabalza-mezghani at ifp.fr
1-4 Av. Bois Preau - Bat Lauriers
92852 Rueil Malmaison Cedex, France


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