title: invalid grouping factor specification in lmer model
body: <p>I am attempting to run a mixed effects model using the lmer function. My experiment included metabolic rates at different temperatures using some of the same individuals (some missing data). The structure of the textfile looks like this:</p>
  
  <pre><code>&gt; str(data.by.animal)
  'data.frame':   18 obs. of  17 variables:
   $ animal: Factor w/ 18 levels "08_03","08_07",..: 17 6 5 10 15 14 11 12 16 9 ...
   $ temp  : int  2 0 -2 -4 -6 -8 -10 -12 -14 -16 ...
   $ X2    : num  0.0129 0.0176 0.0132 NA 0.0144 0.0133 0.0101
  </code></pre>
  
  <p>When I run the script <code>[model_1 &lt;- lmer(X2 + X0 + X.2 + X.4 + X.6 + X.8 + X.10 + X.12 + X.14 + X.16 + X.18 + X.20 + X.22 + X.24 + X.26 ~ temp + (1 | animal), data.by.animal)]</code> I get the following: <code>[Error in FUN(X[[1L]], ...) : Invalid grouping factor specification, animal]</code> and despite consulting "The R Book" and other answers here, I'm still at a loss as to where I'm going wrong.</p>