title: Mean by factor by level
body: <p>Maybe this is simple but I can't find answer on web. I have problem with mean calculation by factors by level.
  My data looks typicaly:</p>
  
  <pre><code>factor, value
  a,1
  a,2
  b,1
  b,1
  b,1
  c,1
  </code></pre>
  
  <p>I want to get vector A contains mean only for level "a"
  If I type A on consol I want to get 1.5
  And this method for calculating mean, must use factors.</p>
  
  <p>Thank you in advance for help.  </p>