Manuel, in order to save the changes you have to assign it to the data
frame.
Here is how you should do it.

nidprop$treatment <- relevel(nidprop$treatment, ref="c", data=nidprop)

Luciano Selzer

2010/5/18 Manuel Spínola <mspinola10@gmail.com>

> Dear list members,
>
> I am trying to change the reference level of a factor but when I run a
> model with the lm function the new order it does not taking place (I still
> have the old order of the levels).
>
> > relevel(nidprop$treatment, ref="c", data=nidprop)
> [1] a a a a b b b b c c c c
> Levels: c a b
>
> > modelo1 = lm(hight ~ treatment, data = nidprop)
>
> > summary(modelo1)
>
> Call:
> lm(formula = hight ~ treatment, data = nidprop)
>
> Residuals:
>   Min      1Q  Median      3Q     Max
> -1.6052 -1.0626 -0.0950  0.9228  3.0768
>
> Coefficients:
>              Estimate Std. Error t value Pr(>|t|)  (Intercept)     37.9140
>     0.7368  51.461 1.98e-12 ***
> treatment[T.b]  -0.2298     1.0419  -0.221   0.8304  treatment[T.c]
>  -3.1133     1.0419  -2.988   0.0153 *
> ---
> Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
>
> Residual standard error: 1.474 on 9 degrees of freedom
> Multiple R-squared: 0.552,      Adjusted R-squared: 0.4525
> F-statistic: 5.545 on 2 and 9 DF,  p-value: 0.02696
>
>
> Any idea why?
>
> Best,
>
> Manuel Spínola
>
> --
> Manuel Spínola, Ph.D.
> Instituto Internacional en Conservación y Manejo de Vida Silvestre
> Universidad Nacional
> Apartado 1350-3000
> Heredia
> COSTA RICA
> mspinola@una.ac.cr
> mspinola10@gmail.com
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>
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