[R] coef and vcoc for polr inconsistent??
John Fox
jfox at mcmaster.ca
Sun Jun 6 01:15:07 CEST 2004
Dear Bendix,
vcov() is indeed giving you the covariances among the coefficients and
inter-level cutpoints, as you can see by taking the square-roots of the
diagonal entries of the covariance matrix that it reports and comparing
these with the values in the table provided by summary() -- or was your
point that coef() doesn't report the cutpoints?
I hope that this helps,
John
> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of BXC
> (Bendix Carstensen)
> Sent: Saturday, June 05, 2004 2:51 PM
> To: r-help at stat.math.ethz.ch
> Subject: [R] coef and vcoc for polr inconsistent??
>
> Is the following an inconsistency, programming glitch or a feature?
> One would expect that vcov(obj) was the variance-covariance
> of coef(obj), but apparently this is not the case for polr objects:
>
> > x <- rnorm( 100 )
> > y <- rnorm( 100 )
> > ff <- factor( sample( 1:4, 100, replace=T ) ) pm <- polr(
> ff ~ x + y )
> > coef( pm )
> x y
> 0.21219010 0.03558506
> > vcov( pm )
>
> Re-fitting to get Hessian
>
> x y 1|2 2|3 3|4
> x 0.033462790 -0.002414745 -0.006183451 -0.004164503 -0.001585546
> y -0.002414745 0.032953020 0.003821232 0.003645342 0.003756031
> 1|2 -0.006183451 0.003821232 0.060134608 0.029651508 0.018105699
> 2|3 -0.004164503 0.003645342 0.029651508 0.041604224 0.025465751
> 3|4 -0.001585546 0.003756031 0.018105699 0.025465751 0.051576286
>
> But of course you CAN get wat you want:
>
> > summary( pm )$coef
>
> Re-fitting to get Hessian
>
> Value Std. Error t value
> x 0.21219010 0.1829284 1.1599628
> y 0.03558506 0.1815297 0.1960289
> 1|2 -1.29481930 0.2452236 -5.2801580
> 2|3 -0.22054056 0.2039711 -1.0812342
> 3|4 0.98703785 0.2271041 4.3461907
>
> > version
> _
> platform i386-pc-mingw32
> arch i386
> os mingw32
> system i386, mingw32
> status
> major 1
> minor 9.0
> year 2004
> month 04
> day 12
> language R
>
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