This is not a meaningful metric. It is meaningful though that no QB/HC combo since the 70;s has made their first Super Bowl after year 4. There are lots and lots of statistical samples for that particular metric. There is a first time for everything, of course. But most head coaches when given a truly elite QB talent, get there pretty promptly. Some like Dan Reeves (to me the best McDermott comp) never get there with their super star QB. Guys like Marty Shottenheimer or Marvin Lewis really never got that super star QB.