title: Dplyr to summarize columns
body: <p>I have a dataset</p>
  
  <pre><code>company_category_list Cluster
  Biotechnology         1
  Software              2
  Biotechnology|Search  1
  Biotechnology         1
  Biotechnology         1
  Enterprise Software   3
  Software              2
  </code></pre>
  
  <p>I want to get a count of the 1st column grouped by the column Cluster so used the following code:</p>
  
  <pre><code>library(dplyr)
  CountSummary &lt;-SFBay_2012 %&gt;% 
  group_by(Cluster) %&gt;% 
  summarise(company_category_list_Count = count_(company_category_list))
  </code></pre>
  
  <p>But getting the following error:</p>
  
  <pre><code>Error: no applicable method for 'group_by_' applied to an object of class "factor"
  </code></pre>
  
  <p>Can anybody help out? 
  Thanks in advance!!</p>