If you’ve ever seen the musical Rent, you will have heard the most recognizable song from it, “Seasons of Love”. The main part of this song poses the question: “How do you measure, measure a year?” The cast and chorus offer such suggestions as daylight, sunsets, midnights, cups of coffee, inches, smiles, laughter, and strife.

What the heck, composer and lyricist, Jonathan Larson? What about in expected points?

The question I have for you all is, how do you measure a career? We’re a few short weeks away from the 2015 Hall of Fame class being announced and coaches Tony Dungy, Jimmy Johnson, and Don Coryell are up for induction. We are seeing two great coaches of today going toe-to-toe in the Super Bowl this year, between Seattle’s Pete Carroll and New England’s Bill Belichick. With all the coaching turnover right now, too, a question on everyone’s mind –- especially one numberFire commenter’s –- is who have the best coaches in the league been?

By pulling up Net Expected Points (NEP) data for all 15 years we've had it (2000 is our main reference), we can measure exactly how good every head coach in the NFL has been in the new millennium.

Who are the best coaches in NEP history?

More Than a Zero Sum Game

Like I said, I went into our NEP annals and matched up every NFL head coach in the last decade and a half with their teams. We measure teams and coaches by both NEP and Defensive NEP, and the most telling version of these is Adjusted NEP –- adjusted for strength of opponent. Which head coaches have had the best careers by these metrics?

The table below shows the top five coaches in the league by their total Adjusted NEP (offense) over the course of their careers. Longevity is a great test of excellence, as those who sustain value on their teams have weathered the test of time in this constantly-changing league. NEP is a cumulative metric, so who has been able to achieve the most offensive success in their time?

Bill Belichick is the only head coach to have remained a head coach with one team for the entire 15 seasons. That kind of stability has led to great success, and Belichick is by far the best coach in terms of a sum of Adjusted NEP over the years. Not far behind him, however, are the trio of nine-year coaches of Sean Payton, Tony Dungy, and Mike McCarthy. It’s no small wonder that the four quarterbacks currently considered the best of their generation are represented at the top of this ranking, in Tom Brady, Drew Brees, Peyton Manning, and Aaron Rodgers.

The next table flips the script on the sides of the ball. Who accumulated the most total Adjusted Defensive NEP in the last decade and a half? Remember, Defensive NEP describes how few points a defense held a team to versus expectation; the lower the number (or more negative), the better.

The tables have been flipped. In Adjusted Defensive NEP, Brian Billick’s Super Bowl-winning, elite defensive units show their prowess, as does Lovie Smith’s. Of the other three, however, two are highly offensive-minded coaches who likely had to call on coordinators to run their defenses: Andy Reid and Jon Gruden. Although Billick had a good eye for personnel and built his defenses, like him they were smart enough to recognize their flaws and bring in coordinators who could do the job well.

Average? Hardly.

Raw totals hardly tell the whole tale, however. Some coaches have had much shorter careers and still made a huge impact on their teams and the league as a whole. Averages can also be a great way to assess team production and really break down different sample sizes for coaches’ careers.

The table below shows the average annual Adjusted NEP for each head coach, displayed for each of the top five in this category. Who was the most efficient on a per season basis on offense?

The top five on this list seems like a rerun of the usual suspects, but with one addition: San Diego Chargers head coach Mike McCoy –- though only in the position for the last two years –- has had the fourth-best average season among coaches in adjusted NEP. Much of this is due to Philip Rivers’ career revival, which is often credited to McCoy as well. Sean Payton's shorter tenure elevates him over the more prolific Belichick here, as they have accumulated a similar total Adjusted NEP, but Payton's offenses did it in a shorter window of time.

The final table shows our best coaches in terms of average Adjusted Defensive NEP. Who has averaged the highest on the defensive side of the ball in their tenures?

Here are some new faces! Sure, Brian Billick's teams have proven to be amazing on the defensive side (shocking, as he was an offensive coordinator for much of his career), and Dave Wannstedt and Jon Gruden were lucky enough to inherit some great defensive units, but there are some others around that may be in the running for the greatest defensive team. Al Groh was Bill Parcells’ linebackers coach with the New York Jets, then when he resigned, Groh was the head coach for just one year. However, his average defensive year was much better than anyone else’s in this span of time. Bobby Ross only has one year in our database, but was the head coach of the Detroit Lions from 1997 to 2000, and fielded a very solid defensive unit in that time, despite the players rebelling against Ross’s strict regimens.

The Best in Show

So, who were the best head coaches in NEP history? It depends on what you believe is the best measure of success in the NFL. If longevity is your cup of tea, Bill Belichick is probably your man on offense, and Brian Billick or Lovie Smith on defense. If averages are the best, Sean Payton is clearly your offensive man, and Al Groh, though his sample size is obviously incredibly limited, is your defensive mind.

As for the best overall, it will always be subjective, but I think it would have to be Bill Belichick. Tony Dungy is a close second, with a higher annual average in Adjusted NEP, a slightly lower average Adjusted Defensive NEP, and lower cumulative totals in both metrics than Belichick. 15 years with the same team, and the most total Adjusted NEP and a top-three placing in average Adjusted NEP sells me pretty well, not to mention Belichick’s average defensive season was a solid -6.38 Adjusted Defensive NEP, the most of the elite offensive coaches.

If I have to pick one winner, it’s New England’s mastermind head coach, the hoody-wearing, everything-strategizing dynasty builder in Bill Belichick. He should be a first ballot Hall-of-Famer whenever he finally retires (if he ever retires); he will certainly be in the NEP Hall of Fame.