# [R] qr and Moore-Penrose

Wed Jun 30 15:44:58 CEST 1999

```> What is a correct result, by the way?  There are infinitely many solutions
> for the regression of y on X, and the Moore-Penrose one is just one choice
> (that assumes that the coefficients are somehow comparable).

hm, 1.5, -0.5, 0.5, 1.5 should be a unique solution to Xb = y (with
minimal 2-norm).

> Those `;' are unnecessary: either CR or ; separates expressions in
> S-like languages.

:-) teachers forced me learing pascal, it's just styling

> > drop(b2)
> [1]  1.5 -0.5  0.5  1.5
> > b1
> [1]  3 -2 -1 NA
> > lm(y ~ X + 0)
>
> Call:
> lm(formula = y ~ X + 0)
>
> Coefficients:
> X1  X2  X3  X4
>  3  -2  -1  NA
> > lm(y ~ X)
>
> Call:
> lm(formula = y ~ X)
>
> Coefficients:
> (Intercept)           X1           X2           X3           X4
>         2.5           NA         -1.5           NA           NA
> [sic]
>
> whereas S gives
>
> > qr.coef(qr(X), y)
> [1]  3 -2 -1  0
> > lm(y ~ X, singular.ok=T)
> Call:
> lm(formula = y ~ X, singular.ok = T)
>
> Coefficients: (2 not defined because of singularities)
>  (Intercept) X2 X3
>            3 -2 -1
>
> Now, I can see the problem with lm under R, but what is wrong with qr.coef?

well ok, that's a problem of lm when using anova design matrix. I found
this phenomenon doing a anova analysis.

Torsten

>
> #--------
>
> In R:
>
> > qr(cbind(1,1,diag(3)))
> \$qr
>            [,1]       [,2]      [,3]       [,4]       [,5]
> [1,] -1.7320508 -0.5773503 -1.732051 -0.5773503 -0.5773503
> [2,]  0.5773503  0.8164966  0.000000 -0.4082483 -0.4082483
> [3,]  0.5773503  0.7071068  0.000000 -0.7071068  0.7071068
>
> \$rank
> [1] 2
>
> \$qraux
> [1] 1.5773503 1.7071068 0.0000000 0.7071068 0.7071068
>
> \$pivot
> [1] 1 3 2 4 5
>
> It seems that there is a problem here (and S gets this right). I think
> the changes to the Linpack pivoting strategy in dqrdc2 fail in this
> example.
>

> --
> Brian D. Ripley,                  ripley at stats.ox.ac.uk
> Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
> University of Oxford,             Tel:  +44 1865 272861 (self)
> 1 South Parks Road,                     +44 1865 272860 (secr)
> Oxford OX1 3TG, UK                Fax:  +44 1865 272595
>
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