title: What is the difference between <NA> and NA?
body: <p>I have a factor named SMOKE with levels &quot;Y&quot; and &quot;N&quot;. Missing values were replaced with NA (from the initial level &quot;NULL&quot;). However when I view the factor I get something like this:</p>
  <pre><code>head(SMOKE)
  # N N &lt;NA&gt; Y Y N
  # Levels: Y N
  </code></pre>
  <p>Why is R displaying <code>NA</code> as <code>&lt;NA&gt;</code>? And is there a difference?</p>