In reply to Rick Wicklin.

Thanks a lot for the code! Visual comparison would be helpful, but I'm looking for a method that can statistically test the model parameters of smoothers across multiple groups. For example, let's consider a 2-way ANOVA design, I want to fit LOESS models for each group and do a statistical test the differences across the groups (instead of comparing group means in a normal ANOVA). Do you have any suggestion for my situation?