[R-sig-ME] [R] understanding I() in lmer formula

Don Cohen don-r-help at isis.cs3-inc.com
Tue Jun 13 21:20:05 CEST 2017

Bert Gunter suggested posting this here:

 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 :
  non-numeric argument to binary operator
when I change this line:
to this:
the complaint goes away.
I'd like to understand why.

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