understanding I() in lmer formula

 Is there a difference between I(x*y) and I(y*x) ?
I have a call to lmer that results in this complaint:
  Error in is.alpha2.subordinate * ~z.min.co.res :

A reproducible example would help ...

In the absence of that, check the classes of the two variables in I() and, if you run the product on the command line, either inside I() or not, what's the class of the result?
If that is order-dependent for the object types you're combining, while I don't know why that might be it would go some way to explaining the outcome.

S Ellison



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