When to park the bus in MLS
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By Kevin Minkus (@kevinminkus)
Should teams park the bus? When?
Goals change games. Garry Gelade recently wrote two excellent pieces on this phenomenon (found here and here). One of his key findings is that teams that are down a goal increase their shooting rates to try to make up the deficit, while teams that are ahead take fewer shots. The thinking goes that teams that are ahead can afford to let up on the attack in order to better maintain defensive shape, and thus give up fewer high quality chances to their opponents. In other words, they park the bus. Whether this is a sound strategy remains an open question, and, if it is, how early is too early to do it?
As an example, here is what the 2014 Crew looked like in terms of shots when behind, tied, and ahead (hat tip to Garry, once again, for the excellent way to visualize this):
Let me know on twitter if you'd like to see a different team's graph for any season from 2011 to 2015 - @kevinminkus.
As you can see, Columbus shot less frequently when in the lead. This is a pretty typical trend.
Using logistic regression, we can evaluate the effect of shots and shot quality on a leading team's chances of conceding the next goal. The model I've built, like Garry's, breaks down a game into a sequence of game states. The game begins at 0-0, and each time a goal is scored, a new game state segment begins. My model takes as inputs the number of shots the leading team takes, and the average quality of those shots (using the site's expected goals model) during a segment. It then outputs the probability of that team conceding the next goal.
In general, teams that shoot more are less likely to concede the next goal in a game. Teams that take better shots are also less likely to concede the next goal. If we include only situations where a team is up by one goal, the same results hold. However, if we only look at time frames towards the end of games and where teams are up by one goal- situations where parking the bus would be appropriate- things change.
To examine the problem this way, I've built separate models using data filtered by when each segment begins. I've filtered the data this way since I'm hoping to answer the question of when a team should start to go into a defensive shell. Using the start time of the segment, I think, is a good though not perfect proxy for this. For example, then, to see whether parking the bus is a good tactic up a goal after 70 minutes, the model is built using data from game segments which begin on or after the 70 minute mark. Note that as a point of interest I've also included whether the leading team is home or away as a variable in the model.
The chart below shows the minute mark I've filtered by, and whether each of the three variables for the leading team- shots, shot quality, and venue (home or away)- has a statistically significant effect on whether that team concedes a goal.
Essentially what this shows is after the 63rd minute, taking more shots no longer decreases a leading team's chances of giving up a goal. If a team is looking to see the scoreline out, this would be the time to implement a tactical change by withdrawing into a defensive shell. It still makes sense, however, to take high quality chances as they come, at least until about the 69th minute.
It's also interesting to note that in close games in the second half, being home or away doesn't really help prevent conceding. This appears to be evidence against teams playing differently up one late at home versus up one late away.
If, instead of holding on to the scoreline, a team's goal is to put the game away by scoring an insurance goal, that can be modeled, too. For the chart below I've built logistic regression models for each minute mark, using the same variables. The output now, though, is the probability the leading team scores the next goal.
The models suggest taking more shots increases a leading team's odds of being next to score until the 71st minute, while taking high quality shots increases a leading team's odds of being next to score until the 77th minute. So, if a team wants one more goal, taking more shots will help until about 19 minutes remaining, and taking high quality shots will help until about 13 minutes remaining.
There's definitely more work to be done in this area. One next step would be to directly evaluate the trade-off between seeing out the score and trying to put the game away by scoring once more.
This analysis also certainly isn't definitive. I've approached the problem this one way, I'm sure there are some flaws with this approach, and I'd love to hear about them, and see other ways to tackle it.