AIC/BIC vs the rule of "must include lower order interaction" I am running a series of mixed effect models, which include both linear and quadratic term of a variable T (continuous) and the main IV I (categorical), and facing a dilemma.
Model 2 include interaction terms with both the linear and quadratic T.
Model 1 include only the quadratic interaction.
With Model 1, I obtained the lowest AIC and BIC. In comparison, for Model 2 the AIC is 5.8 higher and BIC is 26 higher. If I followed the rule of thumb for AIC/BIC-based selection, Model 1 is the one I should select (and the one I preferred).
On the other hand, there's also a lot of discussion regarding the necessity of including the lower order term interaction when quadratic interaction is in the model.
However, it will be very difficult for me to justify the selection of a model that's very unlikely (probability: exp(-5.8/2)=5%) to be better than the one with AICmin. Thus the dilemma.
I appreciate your time.
 A: It's certainly possible to have a situation in which only the interaction is significant. A trivial example is:
> set.seed(12)
> x1 <- rnorm(100,0,1)
> x2 <- rnorm(100,0,1)
> cor(x1,x2)
[1] 0.01592198
> y <- x1*x2 + rnorm(100,0,.3)
> modFull <- lm(y~x1+x2+x1:x2)
> modIntOnly <- lm(y~x1:x2)
> anova(modIntOnly,modFull)
Analysis of Variance Table

Model 1: y ~ x1:x2
Model 2: y ~ x1 + x2 + x1:x2
  Res.Df    RSS Df Sum of Sq     F Pr(>F)
1     98 8.8677                          
2     96 8.7568  2   0.11093 0.608 0.5465
> AIC(modFull)
[1] 50.25384
> AIC(modIntOnly)
[1] 47.51263

The individual x1 and x2 terms clearly add nothing to this model, and have insignificant p-values in the summary of modFull (not shown). The AIC is lower (better) when they are omitted. But now let's say that instead of having available the actual values of x1 and x2 underlying this true model, we only have w1 and w2 that are shifted by exactly 10 units each:
> w1<-x1+10
> w2<-x2+10
> modelShiftedFull <- lm(y~w1+w2+w1:w2)
> modelShiftedIntOnly <- lm(y~w1:w2)
> anova(modelShiftedFull,modelShiftedIntOnly)
Analysis of Variance Table

Model 1: y ~ w1 + w2 + w1:w2
Model 2: y ~ w1:w2
  Res.Df    RSS Df Sum of Sq      F    Pr(>F)    
1     96  8.757                                  
2     98 94.034 -2   -85.277 467.44 < 2.2e-16
> AIC(modelShiftedFull)
[1] 50.25384
> AIC(modelShiftedIntOnly)
[1] 283.6363

Now the individual w1 and w2 terms are critical to the model, with highly significant p-values in modelShiftedFull (not shown). It's the same underlying "reality" in terms of the outcome y, captured identically with respect to AIC by both "full" models. The only difference is that we now have available to us values of the independent variables that have been shifted in location from their "true" values. How sensitive do you want your model to be with respect to this type of problem?
So "must include lower order interaction terms" is more complicated than a "rule." The issue is discussed extensively on this site, for example here and here. I think of it as the generally best way to avoid unexpected problems. Furthermore, unless you are in danger of overfitting, there is little to be lost by including nominally "insignificant" predictors in a model, given the risks of omitted-variable bias. So I would turn this question around: why omit the lower-order terms at all?
One last thought: consider a flexible restricted cubic spline instead of a quadratic fit of your continuous predictor. Then you can readily evaluate the degree of flexibility (number of knots) needed to model your data without imposing a particular parametric form. I suspect that such a model would be superior, by the AIC criterion, to your quadratic fit.
