# Categorical fixed effect w/ 3 levels in LMER

I have a categorical fixed effect with 3 levels that I'm trying to enter into an LME.

a = lmer(Reaction ~ ContinuousEffect * CategoricalEffect + (1|Subject) + (1|StrengthOfStimulus))


StrengthOfStimulus is a numerical variable that's being used as a random effect, along with Subject.

Now, for some reason, when I run this model I only get information on two levels of the CategoricalEffect, when there are really three levels:

Fixed effects:
Estimate Std. Error t value
(Intercept)   6.853550   0.035681  192.08
ContinuousEf -0.005463   0.002046   -2.67
TypeB        -0.213765   0.010707  -19.97
TypeC        -0.198775   0.010595  -18.76


Where A, B, and C are the levels of the CategoricalEffect.

What happened to level A?

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To see exactly how the contrasts are specified, try running contrasts(CategoricalEffect). You should see two columns of contrasts coefficients, where A is set to 0 in each column, and therefore acts as the reference level for both comparisons. – Marius May 8 '12 at 4:12