title: about data merge: the in= option in SAS
body: <p>I'm a little confused by what the following in= step does. </p>
  
  <p>Here is the code: </p>
  
  <pre><code>data data1;
  merge data2 data3 (in=inb);
  by ID;
  if inb;
  run; 
  </code></pre>
  
  <p>I would really appreciate if someone can tell me what the in=inb here does. </p>