title: Why does summarize or mutate not work with group_by when I load `plyr` after `dplyr`?
body: <p><em>Note:</em> The title of this question has been edited to make it the canonical question for issues when <code>plyr</code> functions mask their <code>dplyr</code> counterparts. The rest of the question remains unchanged.</p>
  
  <hr>
  
  <p>Suppose I have the following data:</p>
  
  <pre><code>dfx &lt;- data.frame(
    group = c(rep('A', 8), rep('B', 15), rep('C', 6)),
    sex = sample(c("M", "F"), size = 29, replace = TRUE),
    age = runif(n = 29, min = 18, max = 54)
  )
  </code></pre>
  
  <p>With the good old <code>plyr</code> I can create a little table summarizing my data with the following code:</p>
  
  <pre><code>require(plyr)
  ddply(dfx, .(group, sex), summarize,
        mean = round(mean(age), 2),
        sd = round(sd(age), 2))
  </code></pre>
  
  <p>The output look like this:</p>
  
  <pre><code>  group sex  mean    sd
  1     A   F 49.68  5.68
  2     A   M 32.21  6.27
  3     B   F 31.87  9.80
  4     B   M 37.54  9.73
  5     C   F 40.61 15.21
  6     C   M 36.33 11.33
  </code></pre>
  
  <p>I'm trying to move my code to <code>dplyr</code> and the <code>%&gt;%</code> operator. My code takes DF then group it by group and sex and then summarise it. That is:</p>
  
  <pre><code>dfx %&gt;% group_by(group, sex) %&gt;% 
    summarise(mean = round(mean(age), 2), sd = round(sd(age), 2))
  </code></pre>
  
  <p>But my output is:</p>
  
  <pre><code>  mean   sd
  1 35.56 9.92
  </code></pre>
  
  <p>What am I doing wrong?</p>