title: How to remove groups of observation with dplyr::filter()
body: <p>For the following data</p>
  
  <pre><code>ds &lt;- read.table(header = TRUE, text ="
  id year attend
  1 2007      1
  1 2008      1
  1 2009      1
  1 2010      1
  1 2011      1
  8 2007      3
  8 2008     NA
  8 2009      3
  8 2010     NA
  8 2011      3
  9 2007      2
  9 2008      3
  9 2009      3
  9 2010      5
  9 2011      5
  10 2007     4
  10 2008     4
  10 2009     2
  10 2010    NA
  10 2011    NA
  ")
  ds&lt;- ds %&gt;% dplyr::mutate(time=year-2000)
  print(ds)
  </code></pre>
  
  <p>How would I write a dplyr::filter() command to keep only the ids that don't have a single NA? So only subjects with ids 1 and 9 should stay after the filter. </p>