title: Why are my dplyr group_by & summarize not working properly? (name-collision with plyr)
body: <p>I have a data frame that looks like this:</p>
  
  <pre><code>#df
  ID  DRUG FED  AUC0t  Tmax   Cmax
  1    1     0   100     5      20
  2    1     1   200     6      25
  3    0     1   NA      2      30 
  4    0     0   150     6      65
  </code></pre>
  
  <p>Ans so on. I want to summarize some statistics on AUC, Tmax and Cmax by drug <code>DRUG</code> and FED STATUS <code>FED</code>. I use dplyr. For example: for the AUC:</p>
  
  <pre><code>CI90lo &lt;- function(x) quantile(x, probs=0.05, na.rm=TRUE)
  CI90hi &lt;- function(x) quantile(x, probs=0.95, na.rm=TRUE)  
  
  summary &lt;- df %&gt;%
               group_by(DRUG,FED) %&gt;%
               summarize(mean=mean(AUC0t, na.rm=TRUE), 
                                   low = CI90lo(AUC0t), 
                                   high= CI90hi(AUC0t),
                                   min=min(AUC0t, na.rm=TRUE),
                                   max=max(AUC0t,na.rm=TRUE), 
                                   sd= sd(AUC0t, na.rm=TRUE))
  </code></pre>
  
  <p>However, the output is not grouped by DRUG and FED. It gives only one line containing the statistics of all by not faceted on DRUG and FED.</p>
  
  <p>Any idea why? and how can I make it do the right thing?</p>