title: subset() a factor by its number of observation
body: <p>I have a problem with subset()function. How can I subset a factor of my dataframe by its number of observation?</p>
  
  <pre><code>   NAME      CLASS         COLOR   VALUE      
     antonio       B          YELLOW       5
     antonio       B          BLUE       8
     antonio       B          BLUE       7 
     antonio       B          BLUE      12 
     luca          C          YELLOW    99
     luca          B          YELLOW    87
     luca          B          YELLOW    98
     giovanni      A          BLUE      48
  </code></pre>
  
  <p>I would like to obtain data where the three factors "NAME","CLASS" and "COLOR" compare at least three times in order to make a mean of VALUE. in this case I'll obtain:</p>
  
  <pre><code>   NAME      CLASS         COLOR   VALUE      
     antonio       B          BLUE       mean
  </code></pre>
  
  <p>because antonio is the only with three observations for each factor</p>
  
  <p>thank you so much</p>
  
  <p>Nik</p>