title: select last observation from longitudinal data
body: <p>I have a data set with several time assessments for each participant. I want to select the last assessment for each participant. My dataset looks like this:</p>
  
  <pre><code>ID  week  outcome
  1   2   14
  1   4   28
  1   6   42
  4   2   14
  4   6   46
  4   9   64
  4   9   71
  4  12   85
  9   2   14
  9   4   28
  9   6   51
  9   9   66
  9  12   84
  </code></pre>
  
  <p>I want to select only the last observation/assessment for each participant, but I only have the number of weeks as an indicator for each participant. How is this possible to do in R (or excel?) </p>
  
  <p>thanks in advance, </p>
  
  <p>niki</p>