title: Who came up with the one-third rule?
body: <p>I remember hearing about a year ago about the "one-third rule", which says that if you have to change more than 1/3 of a program you're better off starting from scratch.</p>
  
  <p>Googling around, however, I can't find any references to this rule of thumb, let alone any evidence (even anecdotal), to support it. </p>
  
  <p>Has anyone else heard of this rule and know where it comes from, or did I dream it up myself?</p>