# [R] lm design matrix bug?

Tim Calkins tim.calkins at gmail.com
Mon Oct 29 08:31:22 CET 2007

```check the dimensions of your X and P matrices:

>dim(X)
 21 9
>dim(P)
 9 21

what you see is that the final row is *named* 26.  It's actually row 21.

cheers,

On 10/29/07, toby909 at gmail.com <toby909 at gmail.com> wrote:
> Hi All
>
> Maybe I dont understand it, but I would have expected that the design matrix has
> as many rows as there were observations available to fit the model.
> Below a small artificial dataset created, then one model fitted and the design
> matrix outputted, having 27 rows. Then I delete 6 obs, and fit the model on
> these 21 obs, but the design matrix that comes out has 26 rows?
>
>
> Toby
>
>
>
>
> y = c()
> x1 = c()
> x2 = c()
> idx = 1
> for (i in 1:3) {
>   for (j in 1:3) {
>    for (k in 1:3) {
>     y[idx] = 30*i+10*j+100*i*j+30*k-60
>     x1[idx] = i
>     x2[idx] = j
>     idx = idx+1
>    }
>   }
> }
>
> lm11 = lm(y ~ factor(x1)*factor(x2), x=1)
> summary(lm11)
> unique(predict(lm11))
>
> X = lm11\$x; X
>
> P = solve(t(X)%*%X) %*% t(X); round(P,3)
>
>
> y = NA
> y = NA
> y = NA
> y = NA
> y = NA
> y = NA
>
> lm21 = lm(y ~ factor(x1)*factor(x2), x=1)
> summary(lm21)
> unique(predict(lm21))
>
> X = lm21\$x; X
>
> P = solve(t(X)%*%X) %*% t(X); round(P,3)
>
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