title: missing factor level in R
body: <p>I have a factor column, and I get a missing level, why R creates this missing level?</p>
  
  <pre><code>d0s$y
   [1] E E E E E E G G G G G G G P P P P P P P
  Levels:  E G P
  
  levels(d0s$y)
  [1] ""  "E" "G" "P"
  </code></pre>