title: reshape2 melt warning message
body: <p>I'm using <code>melt</code> and encounter the following warning message:<br>
  <code>attributes are not identical across measure variables; they will be dropped</code></p>
  
  <p>After looking around people have mentioned it is because the variables are different classes; however, that is not the case with my dataset. </p>
  
  <p>Here is the dataset: </p>
  
  <pre><code>test &lt;- structure(list(park = structure(c(1L, 1L, 1L, 1L, 1L, 1L, 1L, 
  1L, 1L, 1L), .Label = c("miss", "piro", "sacn", "slbe"), class = "factor"), 
      a1.one = structure(c(3L, 1L, 3L, 3L, 3L, 3L, 1L, 3L, 3L, 
      3L), .Label = c("agriculture", "beaver", "development", "flooding", 
      "forest_pathogen", "harvest_00_20", "harvest_30_60", "harvest_70_90", 
      "none"), class = "factor"), a2.one = structure(c(6L, 6L, 
      6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L), .Label = c("development", 
      "forest_pathogen", "harvest_00_20", "harvest_30_60", "harvest_70_90", 
      "none"), class = "factor"), a3.one = structure(c(3L, 3L, 
      3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L), .Label = c("forest_pathogen", 
      "harvest_00_20", "none"), class = "factor"), a1.two = structure(c(3L, 
      3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L), .Label = c("agriculture", 
      "beaver", "development", "flooding", "forest_pathogen", "harvest_00_20", 
      "harvest_30_60", "harvest_70_90", "none"), class = "factor"), 
      a2.two = structure(c(6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 
      6L), .Label = c("development", "forest_pathogen", "harvest_00_20", 
      "harvest_30_60", "harvest_70_90", "none"), class = "factor"), 
      a3.two = structure(c(3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 
      3L), .Label = c("forest_pathogen", "harvest_00_20", "none"
      ), class = "factor")), .Names = c("park", "a1.one", "a2.one", 
  "a3.one", "a1.two", "a2.two", "a3.two"), row.names = c(NA, 10L
  ), class = "data.frame")
  </code></pre>
  
  <p>And here is the structure:</p>
  
  <pre><code>str(test)
  'data.frame':   10 obs. of  7 variables:
   $ park  : Factor w/ 4 levels "miss","piro",..: 1 1 1 1 1 1 1 1 1 1
   $ a1.one: Factor w/ 9 levels "agriculture",..: 3 1 3 3 3 3 1 3 3 3
   $ a2.one: Factor w/ 6 levels "development",..: 6 6 6 6 6 6 6 6 6 6
   $ a3.one: Factor w/ 3 levels "forest_pathogen",..: 3 3 3 3 3 3 3 3 3 3
   $ a1.two: Factor w/ 9 levels "agriculture",..: 3 3 3 3 3 3 3 3 3 3
   $ a2.two: Factor w/ 6 levels "development",..: 6 6 6 6 6 6 6 6 6 6
   $ a3.two: Factor w/ 3 levels "forest_pathogen",..: 3 3 3 3 3 3 3 3 3 3
  </code></pre>
  
  <p>Is it because the number of levels are different for each variable? So, can I just ignore the warning message in this case?</p>
  
  <p>To generate the warning message:</p>
  
  <pre><code>library(reshape2)
  test.m &lt;- melt (test,id.vars=c('park'))
  Warning message:
  attributes are not identical across measure variables; they will be dropped
  </code></pre>
  
  <p>Thanks.</p>