Yesterday I mentioned that sack differential and turnover differential have a similar correlation to winning.  However, I also said that despite the relatively equal importance of both, teams should focus more on the sack numbers, as that measure is likely much less random (luck-based).  Today I wanted to illustrate that:

First we’ll look at the persistence of both sack differential and turnovers.  We’ll run the same correlation analysis as we have previously done, this time looking at whether one year’s data point is related to the corresponding data point the following year.  If the value is high (absolute value), then the measure is less random, whereas if there is no correlation then it is largely determined by chance (luck).

Here is the chart for sack differential, remember the data set comprises the last 10 seasons for all 32 teams. Note that in this analysis I have not used 2012/2013 pairs because, obviously, that data doesn’t yet exist.  The Y-axis is sack differential one year, the X-axis is sack differential the following year.

I’ve included the trend line because without it it’s tough to see the relationship.  The correlation value is .29.  Not huge, but significant.  However, by itself it doesn’t tell us too much.  We’ll look at turnover differential persistence now, then we can compare the two.

The value here is .11.  So there is some persistence here, but not nearly as much as we saw in the sack differential.  This tells us that although both turnovers and sacks are very important when it comes to winning, sacks are much less luck-based, meaning there is a lot more teams can do to effect the measure.  Given the choice, teams should look to improve line-play over trying to force/prevent turnovers. (I realize they’re not mutually exclusive concepts, we’re operating in a vacuum here).

A common theme in this blog is how important the efficient allocation of limited resources is when it comes to putting together a football team.  It’s vital that teams understand what is luck and what isn’t, so they know where to focus their efforts in order to maximize the impact of those efforts.

I’m assuming little of this is very surprising to readers here.  We can, however, take the analysis one step further.  Sack differential (and turnover differential) includes both offensive and defensive numbers.  What happens if we break them out?

I looked at both sacks and sacks allowed, to see if there was a noticeable difference in persistence.  First the charts, then the numbers, then interpretation.

Here are the values:

Sacks: .23

Sacks Allowed: .38

I don’t know about you, but I’m a bit surprised at how large the difference is.  There are a TON of variables to consider here.  Both sacks and sacks allowed can be greatly effected by the QB, scheme, line-play, schedule, etc…  Regardless, the above analysis shows that teams appear to have much greater control over how many sacks they allow as compared to how many sacks they create.  This makes some sense, in that the offense has control of every potential sack situation (QBs have a lot more control than defensive players by virtue of having the ball and being able to throw it away.)

In yesterday’s post, I talked about looking at LOS play as a continuum, rather than viewing the OL and DL as discrete units.  If we continue with that train of thought, and apply the above analysis, it certainly appears as though teams should pay more attention to the OL than the DL.  The goal of preventing sacks on offense is much easier to achieve than the goal of creating them on defense.

Therefore, all other things being equal, I’d much rather have a great OL than a great DL.  This has obvious implications for the Eagles.  There is a general consensus that the team will be looking at either OT or DT with the #4 pick in the draft.  IMO, the analysis here would support leaning towards the OT.  Note that we’d still have to evaluate it in the context of the Default Draft Strategy card I put together a few weeks back (which we’ll do once all the scouting shakes out).

For general reference, not everything I show will be counterintuitive or incredibly surprising.  I believe it’s important, though, to illustrate concepts that we take for granted or that have a clear logic to them, if for no other reason then to confirm their validity and hopefully get a better sense for their magnitude and/or context.

I’m going to spend most of today and tomorrow in a car, so I probably wont be able to get anything up tomorrow (or respond to comments).  Be back on Friday though.  Please do comment, as I think that’s extremely important for both developing ideas and challenging my assertions.  Not everything I do is right (shocking), but if nobody points it out or argues, I probably won’t realize it.

Hope everyone enjoyed the holiday weekend.  I’ve got some random thoughts to get out before they get too stale, followed by a quick look at the importance of sacks:

DRC:  I would have tagged DRC, and it would have been an easy decision.  In my mind, the only possible reason for not doing so is if DRC was actively undermining the coaches.  I know a lot of people have resorted to calling DRC a “locker room cancer” and such, but I just don’t get the sense he was that big of a problem.

Last year the entire team fell apart.  The coaches appeared to be undermining each other. Two were fired midseason and it became clear that the head coach would be let go after the season ended.  Is it really a surprise that this led to a team “quitting” on itself?  I’m not trying to excuse that type of play, but the fact is almost the entire team was guilty, and it’s reasonable to think that in a better atmosphere, DRC will return to somewhat consistent play.

He was very bad for a lot of last year, and clearly is not worth $10.7 million.  However, the Eagles DONT NEED THE CAP SPACE.  It really doesn’t matter how much you’re paying him, since it’s only a one year commitment and it doesn’t preclude the team from doing anything else.  Worst case scenario: he sucks again and is gone after next year.

As it stands, it looks like the Eagles will not bring him back.  That means the team is now looking for an entire starting DB corps.  Maybe in a better line-up Nate Allen is acceptable.  Maybe in a great line-up Kurt Coleman can be covered for.  That still leaves 3 starting spots.  I know Howie has claimed to be fully committed to BPA (best player available) in the draft, but it’s going to be extremely tempting to take someone like Dee Milner.  If that ends up being a reach, then the Eagles have blown a huge opportunity to kick-start the next phase of the franchise.

If they don’t draft Millner (or another top prospect), where is the team going to find 2 starting CBs?  I’ve mentioned before that this is not a 1 year process.  Still, I was hoping the team would use its resources to band-aid a few of the holes it couldn’t fix this offseason.  DRC fits this role perfectly (band-aid).

Draft Watching:  It’s fun to read about all of the draft prospects and compare the “big boards”, but remember, at this point most of it’s useless.  The boards are going to change, some dramatically, so don’t get too caught up in things like “Floyd has passed Star” or “no QB’s are worth 1st round picks”.  We’ll probably get a very different story closer to draft day, after all the interviews are done and teams have done a lot more film work.

I do like looking at the National Football Posts rankings, though.  They claim that their evaluations will not be affected by the combine, only by prospect interviews and character issues from here on in.  They’ve got Nassib ranked #1 overall…(he’s the guy the Eagles likely have their eyes on in the Mike Vick-as-insurance scenario).  If this ranking holds, safe to say he won’t be there for the Eagles in round 2.

When it gets closer to the draft I’ll pull together a “consensus” prospect ranking, and we can use that with our Draft Strategy Chart to look at what the Eagles should do.

Nick Foles:  He will probably be traded.  Again, ignore what the “Eagles sources” are saying.  Given the supposed weakness in this year’s QB draft class, holding on to Foles until draft day and then auctioning him off will maximize his value.  Some team will be desperate for a QB and see their top choice gone.  If things fall the right way, the Eagles can definitely get a 2nd round pick for Foles.  At the very least, he’s should garner a 3rd + a later round pick (and IMO he’s worth a lot more than that).

OG’s in the first round:  I don’t care what Mike Mayock says.  I don’t care if Chance Warmack will immediately be the best guard in football.  Choosing a Guard in the first round is a dramatic misallocation of resources.  The draft is not just about finding prospects who will play.  It’s about maximizing the VALUE of every pick.

Sacks:  This relationship will be fairly obvious, but it’s not a stat I see referenced very often (sack differential). Obviously getting sacked is bad and sacking the other QB is good, but the overall sack differential (sacks-sacks allowed) is HIGHLY correlated with winning.   The only Super Bowl champion in the last 10 years with a negative regular season sack differential were the Ravens this year (-1).  Here is the chart, with Sack Differential on the y-axis (and SB winners in yellow):

The correlation value is .62, a very big number.  For reference, this is about the same as the value we found for TO margin (.64).  However, I’d argue that sacks differential is MUCH easier to effect than TO margin. (I’ll do a persistence post as a follow-up).  The idea here is to view OL and DL play as more of a continuum rather than discrete units.  Overall, you want to win the LOS (line of scrimmage).  That could mean EITHER allowing fewer sacks on offense or creating more on defense (I’m intentionally ignoring the run game here).  Ideally you will be good at both, but it’s the net effect that we are looking for.

So for example, rather than fixate on either the OL or DL in the draft, the Eagles should take whoever will add the most to EITHER unit.  Right now, NT is a bigger immediate need than anything on the OL, but if the team finds an OT that adds more than the available NT’s, it should draft the OT.  Given the choice between a Great OL/Bad DL combo and a decent OL/DL combo, it may in fact be better to go with the first choice, which runs counter to popular draft strategy.  Teams typically focus on improving weaknesses, when they should focus on BIGGEST IMPACT.  So choosing to take an area of strength and make it dominant (say the 49ers taking an OL) may be a better strategy than trying to go from mediocre to good at another position group.  This is of course all relative (and near impossible to accurately quantify), but it’s more support for taking the BPA rather than need.

The last post raised a couple of issues (through the comments and twitter) with the Yards Allowed stat.  Clearly it’s not a perfect measure of defense.  As a result, I am pulling together some other defensive metrics and will run the same analysis, in hopes of shedding some light on the relative importance of defense.

In the meantime though, let’s look at why Yards Allowed is a flawed measure.  Hint: We have a problem we have faced before.  That is, teams that are losing by a lot will throw more often, gaining more yards and distorting the defensive ranking of the winning team.

Before we get to that though, we need to examine just how big the potential problem is.  In my previous post on Pass Play %, I hypothesized that there is some skew, but that when looking at every play run in the NFL every year, relatively few of them are done by teams focused on anything except maximizing points scored, which should minimize the effects on the overall data set.

To get a better look, I put together a new analysis, comparing points scored to the ratio of run/pass yards allowed.  The hypothesis is that teams that score a lot of points will force their opponents to pass more, resulting in a positive correlation between points scored and % of Pass yards allowed.  I expect there to be a positive result, as there’s a pretty logical case for correlation.  The real question is how strong the result is; if it’s weak than my prior statement regarding overall skew effects may still stand, if it’s strong then we have to re-evaluate (and do a lot more work).

Here is the chart.  Note the Y axis is just Pass Yards Allowed/Total Yards Allowed.

Dammit.   The correlation value is .40, so moderate, but definitely a little higher than I expected.  Unfortunately, this means we’ve got some work to do if we want to remove the noise resulting from this issue, which will be my goal for the next couple weeks, interspersed with other topics for which I’ve found interesting data.

We’ll attack it from two angles.  First, as I mentioned at the top, we can use other defensive metrics that indicate strong defense but aren’t as susceptible to the same problem.

Also, we can try to use a better metric for overall defense.  There are some interesting stats out there on sites like Football Outsiders that we might be able to use.  I’ve got a few in mind but let me know if you have any particular favorites.  Let me caution you though, stats that look really complicated my be subject to over-fitting, which would lead to really powerful results in analyses like the one above but actually tell us very little useful information.

We can also try to create our own, though I’ll only do that if people are really interested.  The idea of a relatively simple stat that incorporates some major defensive measurables sounds intriguing though.  If you’ve seen something like this, let me know so I don’t waste time duplicating it.

Over at Igglesblitz today, Tommy Lawlor mentioned that QB play and coaching are much more important than people realize, specifically citing the Packers defensive performance and record for 2010 and 2011.  Basically, the defense got A LOT worse while the record got better.

Here’s the link.

I figured I’d add some stats to the point to further illustrate it, because it’s both true and very important.  Unable to quantify coaching, we’ll focus on the QB.

Here are the charts for both Passer Rating vs. Wins and Yards Allowed vs. Wins.

If you look at the slope of both lines (really the absolute value of each), its clear the above slope is much greater, hence a greater correlation.  Note: Tried my best to un-bias the effects of the different scales so it’d be visually clear.  Here are the correlation values:

Passer Rating:  .67

Yards Allowed: -.376

This is particularly relevant given the Eagles current situation, with QB uncertainty and a defense in need of serious upgrades (or significant improvement in current players).  In general, the play of Vick/Foles/Rookie? will be MUCH more important than the performance of the defense next year.

Another somewhat counterintuitive truth (at least regarding the magnitude of the difference in importance).  It also means the Eagles could use this draft, free agency, and scheme change to engineer a dramatic defensive improvement and still end up out of contention if the QB play isn’t good.