title: R doesn't show data when reading a data frame
body: <p>I have a data frame (in excel format) that I'm trying to read into R.</p>
  <p>When I use the view function I can see that some columns show only TRUE/FALSE instead of the numeric variable I can see in the excel file I have.</p>
  <p>Why does it happen and how can I fix it?</p>
  <p>Thanks!</p>