[Rd] Why does poly work for unordered factors?

[Deepayan Sarkar]

On Wed, 22 Oct 2025 at 14:41, Martin Maechler
<maechler using stat.math.ethz.ch> wrote:
>
> >>>>> Roland Fuß via R-devel
> >>>>>     on Wed, 22 Oct 2025 10:24:07 +0200 writes:
>
>     > This doesn't seem intended.
>
> You are right.  The code change, reverting to previous behaviour
> notably for "Date",
> was prompted on this R-devel list,
>     https://stat.ethz.ch/pipermail/r-devel/2022-July/081850.html
>
> But that the change allows poly(<factor>, .) to work was overlooked (by
> me and anyone else ..) and is a bug we will change.
>
>     > See:
>
>     > https://stackoverflow.com/questions/79795583/why-does-poly-work-for-unordered-factors-it-previously-did-not-work
>
> As was already raised in the above SO thread,
> what should happen for *ordered* factors is less obvious.
> A warning was proposed, but I thought that this was too harsh;
> hence, we could use message(), or just keep allowing it.
>
> Opinions?


Given that we use contr.poly by default for ordered factors, I think
it's very natural to allow it (without even a message). In fact, it
would be a nice way to illustrate what contr.poly does; e.g.,

> y <- rnorm(100); g <- gl(5, 20, ordered = TRUE)
> summary(lm(y ~ g)) |> coefficients()
               Estimate Std. Error     t value  Pr(>|t|)
(Intercept)  0.138970785  0.1020089  1.36233970 0.1763120
g.L         -0.182590696  0.2280989 -0.80048932 0.4254247
g.Q         -0.206493256  0.2280989 -0.90527968 0.3676074
g.C          0.003626904  0.2280989  0.01590058 0.9873471
g^4         -0.074807753  0.2280989 -0.32796199 0.7436621
> summary(lm(y ~ poly(g, 4))) |> coefficients()
              Estimate Std. Error     t value  Pr(>|t|)
(Intercept)  0.13897078  0.1020089  1.36233970 0.1763120
poly(g, 4)1 -0.81657042  1.0200891 -0.80048932 0.4254247
poly(g, 4)2 -0.92346592  1.0200891 -0.90527968 0.3676074
poly(g, 4)3  0.01622001  1.0200891  0.01590058 0.9873471
poly(g, 4)4 -0.33455044  1.0200891 -0.32796199 0.7436621

Best,
-Deepayan

>
> Martin
>
> --
> Martin Maechler
> ETH Zurich  and   R Core team
>
>     > --
>     > Dr. Roland Fuß
>
>     > Thünen-Institut für Agrarklimaschutz/
>     > Thünen Institute of Climate-Smart Agriculture
>
>     > Bundesallee 65
>     > D-38116 Braunschweig, Germany
>
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