Wednesday, January 7, 2009

Does Baltimore's Defense Travel?

by Denis O'Regan

This was supposed to be a short piece on home field advantage,but then I caught Boomer Esiason previewing th AFC wildcard weekend on NASN and it turned into something more.

In discussing the Baltimore Miami game Boomer's view was that the Ravens wouldn't be inconvenienced too much by being on the road because "defense travels".
So I decided to see if defense generally and Baltimore's in particular does travel.

Firstly,I compared Baltimore's home and away record since week one 2004,a decent sample size and a period during which their defense has been their dominant asset.

They are 29-12 at home and 15-25 away.If defense does travel I would expect their home and away records to be a lot closer than they actually are.

So next I used a method that gives a reasonable estimation of a team's home field advantage.I'll outline the ideal (but wholly impractical) methodology used and then try to show how you can get reasonable approximations using readily available data.

Say you've got two teams,we'll call them S and D,S can be the superior team.The first step towards determining HFA for each would be to determine how much better,on average in terms of points S is compared to D at a neutral venue.To do this we'd simply require S to play D,over and over at a neutral spot and average the margin of victory (or the occasional defeat).

We'd then ask the teams to repeat the exercise,with D acting as the hosts.After enough re runs,we would again average the margin of victory or defeat for S and could infer that the amount that this figure had declined compare to S's superiority on neutral turf,would be equal to D's HFA.

Similarly when we played out an extended series at S's home field,the amount that the average margin of victory had increased compared to the neutral figure would be as a result of S's HFA.

Of course none of he above is possible.For a start,aside from the odd trip to London and the Superbowl,hardly any neutral venue games are played.

It does give  clue as to how to approach the problem though.If you take the average difference between the margin of victory with S hosting D and D hosting S,you've got a figure that comprises the HFA of D plus the HFA of S.

There are still problems,even for divisional rivals to get,say 40 pairs of games,you're looking at 40 seasons of results.Teams change,as do likely contributing factors that could affect HFA.

So instead do the next best thing and take,say San Diego's last 40 home and away games (5 years of games).The average margin of victory at home was 10.9 points,away from home it was +3.8 points.That's a spread of 7.1 points.

SD's opponents over those 40 games should represent a fair cross section of the NFL,as should the HFA's of those teams.Therefore,as a good approximation for the HFA of SD's opponents we could use the average HFA for the league over the same period.That figure is 2.5 points.

So subtracting 2.5 from 7.1 gives us SD's HFA over the last five years as 4.6 points.

Do the same for every team and you get.

Team.......HFA (points).
Baltimore    .......8
St Louis..........4.7
San Diego.......4.6

So over the period Baltimore,as their win/loss record suggests appear to have benefited enormously from being at home and have fared comparatively poorly on the road.Rather than defense travelling well,the reverse could actually be true.

The next step towards perhaps proving that defense doesn't travel involves using a statistics that can represent a teams defensive capability.I used FO's dvoa stats.Their website contains weekly offensive,defensive and special teams dvoa stats from 2004 onwards.

I recorded the offensive,defensive and ST dvoa each team and their opponents took into every game from week 4 2004 to week 16 2006.I regressed these variables against the actual game result.I then ran a real time live test through the 2007 and 2008 season,using the regression line to predict game outcomes.

In short the respective dvoa stats that the home and away teams took into a match up are statistically significant in predicting the outcome of that match up.

The difference between the margin of victory (or defeat) predicted by the regression line and the actual margin averages out at 10.6 points per game over the two seasons of live testing.By contrast the Vegas line is out by on average 10.5 points per game over the same time scale.However,the dvoa based regression shades Vegas overall by being closer to the actual margin of victory in 60% of games.The slightly better average margin of error by Vegas compared to dvoa could be explained by Vegas incorporating readily available team news.

Having,hopefully established the legitimacy of the dvoa  regression line we can now use it to see if it says anything about Baltimore's defense.

To see which of the individual dvoa stats are most important in determining game outcome I standardised the inputs and re did the regression.

The largest contributer to match outcome is the offensive dvoa of the away side,followed by the offensive dvoa of the home side.Because the regression contains a constant that equates to the average home field advantage for the NFL as a whole this tells you that having a good offense helps a team more on the road than at home,but the gap is small.

The third biggest contributer is home defensive dvoa,but it is followed by the away special teams dvoa and the home special teams dvoa.The smallest contributer to game outcome is the defensive dvoa of the away side.

This is significant to the Baltimore case.The offensive and special teams dvoas merely tweak the built in home field advantage because the relative sizes of their home and away regression coefficients are similar in size.But if you rely greatly on defense,as Baltimore do,the fact that the size of the home and away defensive regression coefficients differ greatly results in a comparatively good predicted home performance and a comparatively poor away one.

So does this predicted home away split occur in practice.

Given the home/away win/loss splits and the apparently large home field advantage,it appears to.But just to be sure,I split Baltimore's 2008 games by venue and looked at the average yards per play allowed through the air and on the ground by the defense.I didn't correct for opponent.

At home the Ravens allowed 4.15 yards per pass and 3.34 yards per run.Those figures increased to 6.12 ypp (increase of 47%) and 3.76 ypr(increase of 13%) on the road.

Spread over the 5 seasons from 2004 onwards,this drop off in defense on the road is still present,albeit less dramatic.Overall Baltimore yields 10% more yardage both to the run and the pass on the road compared to at home.

So to summarise,using dvoa ratings as a measure of team talent,and regressing those ratings against game outcome implies that a good defense is very helpful at home,but performs comparatively poorly on the road.

The really interesting question is why if,Baltimore are typical,do defense reliant teams struggle on the road compared to at home.An obvious cause could be that it's easier for a ref to call pass interference against a road side,but for the moment that's just speculation.Penalty stats are also notoriously difficult to accurately collect.

Another point to consider is that teams that have large home field advantages (for whatever reason,climate being an obvious other factor) are going to be inconvenienced at some stage during the post season.Baltimore are rightly considered to be slight favs even on the road in Miami,but it may be no coincidence that if we look back at the hfa table,all of the Superbowl winners over the period considered had little or no home field advantage.


Anonymous said...

This is a very interesting analysis of HFA. Of course, the teams with the lowest HFA might be the best road teams, just a different way of looking at the same numbers.

Anonymous said...

One more minor thing. If the league average HFA is 2.5, shouldn't the total of the HFA for all 32 teams be 80 ? The total of the numbers presented is 76.7 which is a little low, even accounting for rounding and truncating.

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