title: Significance of 'I' keyword in lm model in R
body: <p>I was creating a linear model for my assignment :</p>
  
  <pre><code>lm(revenue ~ (max_cpc - max_cpc.mean), data = traffic)  
  </code></pre>
  
  <p>But it throws: </p>
  
  <pre><code>Error in model.frame.default(formula = revenue ~ (max_cpc - max_cpc.mean),  : 
     variable lengths differ (found for 'maxcpc.mean') 
  </code></pre>
  
  <p>Then, through trial and error, I slightly modified my code :</p>
  
  <pre><code>lm(revenue ~ I(max_cpc - max_cpc.mean), data = traffic)
  </code></pre>
  
  <p>and Bingo!!!It worked well.</p>
  
  <p>But now I am trying to figure out the significance of 'I' and how it fixed my problem. Can anyone explain it to me?</p>