[R-sig-eco] Output for interactions in models that do not include all main effects

[Kristen Gorman]

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