title: Get the means of sub groups of means in R
body: <p>I'm a newbie of R and I don't know how to get R calculate the means of a subgroups of means which are the means of a subgroup themselves. I'll explain clearer.</p>
  
  <p>I have a data frame like this:</p>
  
  <pre><code>GROUP WORD WLN
  1     1    4
  1     1    3
  1     1    3
  1     2    2
  1     2    2
  1     2    3
  2     3    1
  2     3    1
  2     3    2
  2     4    1
  2     4    1
  2     4    1
  ...   ...  ...
  </code></pre>
  
  <p>but the real one has a total of 5 groups and 25 words (5 words each group; every word has being assigned a number from 1 to 4 by 5 subjects...).</p>
  
  <p>I need to get the means of WLN for every word and I can do that easily with a loop and save the results in a vector; but then I need a vector with the means of these means according to the group which the words belong to... So I need the means of means of words of the group 1, then of group 2, etc... (I don't know if I'm making it clear).</p>
  
  <p>How can I get this without doing it one group by one?</p>