Collectively, NFL franchises are terrible at distinguishing the good head coaching candidates from the bad. Every January, we see the same group of prospective coaches — coordinators of the league’s best offenses and defenses, former greats looking to latch on with their second or third team, and a few big-name college coaches — linked to every job opening. Of those eventually chosen, nearly two-thirds fail to produce winning records over the course of their tenure.

If NFL franchises know little about identifying successful head coaches, as a football community we know far less. NFL coaching is an iceberg. Ninety-eight percent of it is completely invisible to those outside of the organization. What happens on Sundays from September to January is just a small part of the job title. Late-summer hours on the practice field, early-morning meetings at team facilities, long nights in the film room — these are the situations when coaching’s most important job functions occur.

With the 49ers in the midst of a sprawling search for the franchise’s 19th head coach, I wanted to see if there were any quantifiable traits that we could identify before a head coach was actually hired that might shed some light on the candidates that Jed York and Trent Baalke should be focusing on.

I went back and looked at the tenure of every head coach hired since the advent of the salary cap in 1994 with a minimum of 16 games coached — 145 coaching tenures in all — taking note of a number of different factors. Let’s run through a list of the questions I had when starting the process and see if we can’t come up with some sort of profile for what a successful NFL head coach looks like leading up to the moment he’s actually hired.

Who has more success: offensive-minded or defensive-minded head coaches?

Considering how the 2014 season went for the 49ers, the first thing I wanted to know was whether it was better to hire a head coach with an offensive or defensive background. Perhaps more accurately, did it even matter? My gut told me it was better to hire an offensive-minded head coach considering the league’s evolution into an offense-friendly, pass-happy game over the past couple of decades, but I had no idea as to whether there was anything to support that.

As it turns out, offensive-minded head coaches have been more successful, but only by a small margin. Head coaches with an offensive background hired since 1994 have produced a Pythagorean win percentage of .498, slightly ahead of the .486 mark posted by coaches with a defensive background. (Note: I’m using Pythagorean win percentage rather than actual win percentage because the relationship between points scored and points allowed is a more accurate reflection of team performance than straight wins and losses. But from what I found, you can draw similar conclusions from either.)

That’s hardly damning evidence against hiring defensive coaches. The two most successful head coaching tenures in the sample — Bill Belichick’s run with the Patriots (.702) and Tony Dungy’s tenure with the Colts (.686) — both came from defensive-minded frontmen. At the opposite end of the spectrum, seven of the eight worst coaching tenures by Pythagorean win percentage were offense-first coaches.

If we look only at coaching hires in the past decade, the gap between offensive and defensive coaches does widen a bit. Since 2005, offensive coaches have produced a Pythagorean win percentage of .486; defensive coaches have put together a .457 mark. That’s certainly a significant difference, but I’m not sure it’s enough to say hiring an offensive-minded head coach is clearly a superior option.

As an aside, there has been just one head coach hired in our sample with a predominantly special teams background: John Harbaugh. The "other" Harbaugh (at least around these parts) has obviously had a great run of success in Baltimore, and Chase Stuart of Football Perspective made an interesting argument as to why a well-qualified special teams coach could possibly be a better hire than the "whiz kid" offensive and defensive coordinator types.

How much does age or prior NFL experience matter?

The candidates connected to the 49ers’ head coaching vacancy have been all over the place, ranging from wunderkind coordinators (Adam Gase and Kyle Shanahan) to cagey vets that have been around the NFL block a time or four (Mike Shanahan and Vic Fangio). But are either more likely to produce on-field success?

Not at all. Across the entire sample, neither age nor prior NFL experience have any correlation with Pythagorean win percentage as a head coach. Mike Tomlin and Jon Gruden were both hired at the age of 35 and are among the 15 most successful coaches in our sample, while Lane Kiffin and Marty Mornhinweg are examples of whiz kid coordinators gone wrong. Several head coaches hired in their late–50s have had success recently, including Andy Reid, John Fox, and Pete Carroll, while the five-worst head coaching tenures in the sample were from coaches hired after the age of 50.

If there’s anything resembling an age sweet spot, it would be coaches hired in their 40s. Head coaches hired in their 40s have a Pythagorean win percentage of .520, giving them a solid margin over any other age bracket (the next closest are coaches hired in their 30s with a .487 Pythagorean win percentage).

Intuitively, this makes some sense. Coaches in their 30s are still figuring things out; coaches in their 50s, and especially 60s, are often too rooted in outdated principles to change with the shifting trends of the league. But by the time your average coach hits his 40s, he’s been in the league around a decade, give or take a few years. Most have had an opportunity to work under a few different head coaches, pulling some best practices from each while building their organizational philosophy. It also feels like coaches in their 40s would strike the right balance between having a solid foundation of proven strategies while being open-minded enough to implement new-age techniques when appropriate.

Are retreads more successful than first-time head coaches?

If you happened to have listened to the Better Rivals podcast following the conclusion of the regular season, you know that I wasn’t exactly enthralled with the idea of hiring one of the many retread candidates that pop up each year. My basic premise: give me the exciting unknown of the wunderkind coordinator over the stale, been-there-done-that feeling of the retread. But is there anything that actually supports this is a good idea?

With a Pythagorean win percentage of .515, retreads have a leg up on their peers making the leap to head coach for the first time. In fact, on average head coaches actually have more success with each passing job. First-time coaches have a Pythagorean win percentage of .482. That number improves to .506 at their second stop before jumping up to .542 on their third job or later, giving a clear advantage to selecting a candidate with prior NFL head coaching experience.

Findings from Andrew Healy at Football Perspective reaches a similar conclusion. Healy’s research found that first-time head coaches carry over little value from their performance as a coordinator while previous head coaches carried over about 70 percent of their value from previous jobs. In short, being a successful head coach involves far more than playcalling or developing an offensive gameplan and those who find success in that role are likely to have continued success at their next stops.

This makes some intuitive sense as well. Coaches that fail in their first opportunity are less likely to be hired in the future. Those who showed enough promise to get a second crack at it either learn from their previous failures and turn the corner or wash out again, relegating themselves to career-assistant status. Of the 145 coaches in our sample, only Norv Turner managed to get a third head coaching job after posting a Pythagorean win percentage below .500 in each of his first two tenures.

However, there are some other interesting aspects to consider here. Candidates looking for their first head coaching gig often aren’t in a position to be very selective and many end up in hopeless situations with dysfunctional organizations where’s there’s little chance any coach would be able to turn things around. Coaches looking for job no. 2 are often among the highest-paid assistants thanks to their prior stints as head coaches, and are in better position to wait for a more ideal situation to present itself. But more on this in a bit.

What do the most successful head coaches have in common?

If you find yourself frustrated with the idea that a candidate’s background has little correlation with his success as a head coach, I’m right there with you. But what if we looked at only the most successful coaches? Do they share any similar traits or backgrounds that point us in the right direction?

I narrowed our sample to coaches that went on to have a Pythagorean win percentage of .560 or higher, while also putting together multiple winning seasons during their tenure. This gave us 27 coaches to work with. Yet, even our top tier of coaches come from a variety of backgrounds and circumstances in the years leading up to their hire.

There’s an even split between coaches with offensive and defensive backgrounds (plus that Harbaugh fellow with the weird special teams background). Same for retreads (14) and first-time head coaches (13). Most were coordinators (17) in their job immediately preceding their hire as head coach, but that’s true of all hires. Interestingly, just two were internal hires — Dave Wannstedt with the Dolphins and Wade Phillips with the Bills.

Using all that we’ve learned thus far, the closest thing we would have to a profile for NFL head coaching success — and it’s really a stretch — would look like this: an offensive-minded coach in his 40s with between 7–14 years of NFL coaching experience that’s looking for his second head coaching job. But again, that’s stretching things quite a bit. As far as I can tell, the profile for a successful NFL head coach doesn’t really exist. The best coaches come from a variety of different backgrounds and experiences, and from the research I’ve seen (combined with my own), there’s very little in the way of measurable data currently available that can help us predict NFL head coaching success with any more reliability than a coin flip.

What do the least successful head coaches have in common?

While we can’t say with any certainty what will predict success among NFL head coaches, there are a few traits that we probably want to avoid.

Hiring position coaches almost never works out. Of the possible jobs a coach can have prior to making the leap to head coach — head coach with another team, coordinator, position coach, or college — position coaches fare the worst by a significant margin. There have been just 14 position coaches to make the jump to head coach in the salary cap era. On average they have produced a Pythagorean win percentage of .451, well below coordinators (.492), head coaches (.509), or college hires (.516).

There have been just two coaches who have made a successful transition from position coach to head coach in one season: Jim Caldwell with the Colts and Jeff Fisher with the Oilers.

Caldwell’s success can be explained with two words: Peyton Manning.

You could make an argument that Fisher doesn’t qualify at all. After spending time as the defensive coordinator for the Eagles and Rams between 1988 and 1991, Fisher took a step back to become the defensive backs coach in San Francisco under George Seifert in 1992, where he remained for two seasons. In 1994, Fisher took another coordinator position, this time in Houston under Jack Pardee. Pardee was fired after 10 games, and Fisher began his 17-year run as the franchise’s head coach.

Outside of those two, it’s a pretty terrible lot that includes the likes of Mike Singletary, Jim Zorn, and Rod Marinelli.

The other move that seems to generally be a bad idea are internal hires. Twenty-one of the coaches in our sample were internal hires and their .421 Pythagorean win percentage was the worst split I looked at by a wide margin. Sorry, Jim Tomsula. Being a head coach just might not be your thing.

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The more research I did, and the further I dug into the data, the more two things began to stick out to me above all else.

First, the factor that plays the largest role in determining the success of a head coach is the situation he’s entering. Remember our stat from the beginning of the article? Nearly two-thirds of head coaches hired in the past two decades failed to produce a record over .500 throughout their tenure. Considering the wide majority of head coaching vacancies open on teams coming off of poor seasons, which seems more likely to you: that two-thirds of the men who reach the peak of the profession are significantly worse than the one-third who do find success? Or that the situations they entered were more difficult to manage?

If Coach A goes to an organization with a competent front office that has acquired quality talent and Coach B goes to a fumbling organization with a propensity for repeating the same mistakes over and over, does it really matter if Coach B is a better head coach? The wide majority of the time, the coach with the more ideal situation is going to experience more success because of factors that have nothing to do with him or his ability to coach.

Which brings me to my second takeaway: NFL head coaches aren’t nearly as important as we think they are. Before you sprint in a rage to the comments, take a breath, and let’s step back for a second.

I’m not saying that coaches are unimportant. I maintain the belief that the three most important positions within an NFL organization are general manager, head coach, and quarterback, in some order. If you had asked me a week ago, head coach probably topped that list. Now? It’s a distant third.

Consider the tremendous amount of variation that we experience every single NFL season. The factors that drive that variation often have little to do with the head coach. Injuries, fumbles, home-field advantage, penalties in crucial situations, and a number of other random processes all play a big role in determining the outcomes of NFL games. All also fall well outside the control of a head coach. If coaches truly had the impact on the bottom line we thought they did, teams would be far more consistent from season to season.

I understand this is not a view many are likely to share. As Brian Burke of Advanced Football Analytics points out when discussing this topic, it’s human nature to attribute group success and failure to leaders. In the NFL, when a team has a terrible season, fires its head coach, and proceeds to improve the following season under new leadership, it’s the head coach (or sometimes, the general manager who hired him) that receives the credit even though regression toward the mean tells us that was likely to happen anyway.

Beyond the additional biases Burke mentions in his analysis, another reason we probably give coaches more credit (or blame) than they deserve is because we have a tendency to focus on the outliers — the truly great and the truly awful. These are the coaches who actually have a more pronounced impact on team success. As a fan-base that recently witnessed possibly the single-biggest coaching upgrade in league history when the 49ers went from Mike Singletary to Jim Harbaugh, we’ve seen firsthand the impact coaches can have.

However, we’re talking about the one or two percent of coaches on each end of the spectrum. The vast majority of NFL coaches fit somewhere between the Walshes and Belichicks at the peak of the mountaintop and the Singletarys and Crennels who can’t get out of their own way at the bottom. As Burke puts it, "NFL outcomes are overwhelmingly driven by player talent and luck."

Regardless of who San Francisco eventually hires to replace Jim Harbaugh, it’s more likely Colin Kaepernick is the one who determines the fate of the 49ers next season and beyond.