[R] problem with interaction in lmer even after creating an "interaction variable"

a_lampei anna.lampei-bucharova at uni-tuebingen.de
Thu Nov 7 12:15:31 CET 2013

Dear all,
I have a problem with interactions in lmer. I have 2 factors (garden and
gebiet) which interact, plus one other variable (home), dataframe arr. When
I put:
lmer (biomass ~ home + garden:gebiet +  ( 1|Block), data = arr)/

it writes:
/Error in lme4::lFormula(formula = biomass ~ home + garden:gebiet + (1 |  : 
  rank of X = 28 < ncol(X) = 30/

In the lmer help I found out that if not all combination of the interaction
are realized, lmer has problems and one should do new variable using
"interaction", which I did:
arr$agg <- interaction (arr$gebiet, arr$garden, drop = TRUE)/

when I fit the interaction term now:
lmer (biomass ~ home + agg+  ( 1|Block), data = arr)/

The error does not change:
Error in lme4::lFormula(formula = biomass ~ home + agg + (1 | Block),  : 
  rank of X = 28 < ncol(X) = 29/

No NAs are in the given variables in the dataset.

Interestingly it works when I put only given interaction like

/lmer (biomass ~ agg +  ( 1|Block), data = arr)/

Even following models work:
/lmer (biomass ~ gebiet*garden +  ( 1|Block), data = arr)
lmer (biomass ~ garden + garden:gebiet  +( 1|Block), data = arr)/

But if I add the interaction term in th enew formate of the new fariable, it
reports again the same error.

/lmer (biomass ~ garden + agg  +( 1|Block), data = arr)/

If I put any other variable from the very same dataframe (not only variable
"home"), the error is reported again.

I do not understand it, the new variable is just another factor now, or? And
it is in the same dataframe, it has the same length.

Does anyone have any idea?

Thanks a lot, Anna


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