Maybe your solution can be found here:
http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-does-the-output-from-anova_0028_0029-depend-on-the-order-of-factors-in-the-model_003f

2012/4/3 Kristen Gorman <kgorman@sfu.ca>

> Dear all,
> I have R code to run AIC including multi-model inference. I am running
> into a problem in calling the output from models where both parameters in
> an interaction are not included as main effects. In R, the interaction will
> be called depending on the parameter that was used as the only main effect
> in the model. So, I end up generating 2 different interactions (e.g.,
> Rlipid:RFGinit vs RFGinit:Rlipid) that are actually the same. This becomes
> a problem in the remaining R code that requires weighted and summed values
> for the parameter and SE estimates. Thus, I would like to call the
> interaction consistently across models. See the following code:
>
> --
> lm(formula = Slipid ~ Rlipid + RFGinit:Rlipid, data = DataSet)
>
> Residuals:
>    Min      1Q  Median      3Q     Max
> -74.075 -19.047   7.233  20.445  45.391
>
> Coefficients:
>                Estimate Std. Error t value Pr(>|t|)
> (Intercept)    120.33847    5.30405  22.688   <2e-16 ***
> Rlipid           0.30493    0.23615   1.291    0.202
> Rlipid:RFGinit  -0.02099    0.01773  -1.184    0.241
> ---
> Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
>
> Residual standard error: 30.88 on 60 degrees of freedom
> Multiple R-squared: 0.02721,        Adjusted R-squared: -0.005221
> F-statistic: 0.839 on 2 and 60 DF,  p-value: 0.4372
>
>
> lm(formula = Slipid ~ RFGinit + Rlipid:RFGinit, data = DataSet)
>
> Residuals:
>   Min     1Q Median     3Q    Max
> -76.35 -21.63   7.09  22.46  45.71
>
> Coefficients:
>                 Estimate Std. Error t value Pr(>|t|)
> (Intercept)    131.028546   8.717104  15.031   <2e-16 ***
> RFGinit         -0.933483   0.742083  -1.258    0.213
> RFGinit:Rlipid   0.003926   0.009283   0.423    0.674
> ---
> Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
>
> Residual standard error: 30.9 on 60 degrees of freedom
> Multiple R-squared: 0.02586,        Adjusted R-squared: -0.00661
> F-statistic: 0.7964 on 2 and 60 DF,  p-value: 0.4556
> --
>
>
> Is there a way to tell R to call the interaction based on alphabetical
> order of the 2 interaction terms and not based on the term that was used as
> a main effect?
>
> Thanks very much for any insight.
>
> Kristen Gorman
>
> _______________________________________________
> R-sig-ecology mailing list
> R-sig-ecology@r-project.org
> https://stat.ethz.ch/mailman/listinfo/r-sig-ecology
>



-- 
Currículo: http://lattes.cnpq.br/7541377569511492

	[[alternative HTML version deleted]]

