title: Difference between complete.cases and !is.na
body: <p>I just discovered this new function it seems to me to an improved version of !is.na, maybe wrapped into an <code>apply(df, 1)</code>. Am I correct or the following:</p>
  
  <pre><code>&gt; a&lt;-c(1,2,4,NA,6,8)
  &gt; identical(complete.cases(a), !is.na(a))
  [1] TRUE
  </code></pre>
  
  <p>it's not always true?</p>