title: Error when running stepwise regression
body: <p>I am trying to run a stepwise regression model. I keep receiving this message:</p>
  
  <pre><code>#Error in step(cdc.fit, direction = "backward") : 
  #  number of rows in use has changed: remove missing values?
  #In addition: There were 50 or more warnings (use warnings() to see the first 50)
  </code></pre>
  
  <p>Am I receiving this because of missing values?</p>
  
  <p>Here is my code:</p>
  
  <pre><code>model=glm(health~
        ALCDAY5+
        AVEDRNK2+
        CHILDREN+
        CHKHEMO3+
        POORHLTH+
        BLOODCHOYes+
        BPHIGH4No+
        CHCOCNCRYes
        , data=data, fmaily=binomial)
  
   stepmodel_back &lt;- model(cdc.fit,direction='backward') 
   summary(stepmodel_back) 
  </code></pre>
  
  <p>Thanks!</p>