title: Sample by group using the sample_n function of dplyr
body: <p>According to the <code>dplyr</code> help file the <code>sample_n</code> function samples a fixed number per group.</p>
  
  <p>When I run the following code I expect two samples per tobgp and alcgp combination, so 32 (4*4*2) lines in total. However only two lines are returned. </p>
  
  <pre><code>by_tobgp_alcgp &lt;- esoph %&gt;% group_by(tobgp,alcgp)
  
  sample_n(by_tobgp_alcgp , 2)
  
  
  # Source: local data frame [2 x 5]
  # Groups: tobgp, alcgp
  # 
  #    agegp     alcgp tobgp ncases ncontrols
  # 10 25-34    80-119 10-19      0         1
  # 50 55-64 0-39g/day   30+      4         6
  </code></pre>
  
  <p>Is this correct? Is there an alternative way to achieve this using <code>dplyr</code>?</p>