title: How to average columns based on ID in R?
body: <p>I want to average the values by their IDs but not all ID's have the same number of values. How do I do this in R?</p>
  
  <p>I have two columns ID and Value</p>
  
  <pre><code>    ID    Value
      1000    0.51
      1000    0.01
      1001    0.81
      1001    0.41
      1001    0.62
      1002    0.98
      1002    0.12
      1002    0.15
      1003    0.12
      ...     ...
  </code></pre>