title: Consistent factor values across multiple columns
body: <p>I have a CSV file with four fields:</p>
  
  <pre><code>ID, choice1, choice2, choice3
  </code></pre>
  
  <p>The three choice fields are strings that represent categorical variables, where the domain is the same in each case.</p>
  
  <p>I want to read this in using R and have these as factors, not strings, but using consistent level values across the columns. So 'foo' in choice1 columns has the same value as 'foo' in choice2 column, etc.</p>
  
  <p>How do I do ensure that the categorical type is the same across all the choice columns?</p>