title: Calculate average time between two datetimes across all records?
body: <p>Each record has a <code>started_at</code> and <code>completed_at</code> datetime field. To calculate the time it took someone to start and complete is obviously easy.</p>
  
  <p>But what I need to do is calculate an average time from start to complete for ALL records.</p>
  
  <p>Ultimately want to be able to say "It takes, on average, X amount of time to start and complete."</p>
  
  <p>I'm running Rails 3.0.6, in case there's some functionality already built in with that.</p>
  
  <p>Also, it's a Postgresql database, but ideally this would work across other databases.</p>