title: Splitting a data.frame by a variable
body: <p>I have data from several subjects stored in a single CSV file. After importing the CSV file, I would like to split the data from each participant off into its own data.frame.</p>
  
  <p>More literally, I would like to take the example data below, and create three new data.frames; one for each of the 'subject_initials' values.</p>
  
  <p><img src="https://i.stack.imgur.com/Utf6t.png" alt="enter image description here"></p>
  
  <p>How do I do this? I've thus far looked into options using the <code>plyr</code> package and <code>split()</code>, but haven't yet found a solution. I know I'm probably missing something obvious.</p>