The theory behind the defensive shell game
Last week there was a twitter discussion on the merits of playing a defensive shell game by limiting scoring chances against but also limiting scoring chances for, even if it meant the ration of goals for to goals against gets worse. The two sides of the debate are as follows:
Argument 1: It is always best to play a game where you are expected to out score the opposition regardless of the goals for/against rates.
Argument 2: When playing with a lead late in the game it is more important to reduce the goals against rate than maintain the goals for rate, even if it means the goals for to goals against ratio drops significantly.
To test each theory I simulated a number of games between teams T1 and T2 according to the following theories:
1. During normal play between teams T1 and T2, T1 will score at a rate of 2.75 goals/60 minutes and T2 will score at a rate of 2.50 goals/60 minutes. During this play it is expected that T1 will score approximately 52.4% of all the goals that are scored.
2. During play between T1 and T2 when T1 has a lead and is playing in defensive shell mode T1 score at a rate of 2.00 goals/60 and T2 will score at the same 2.00 goals/60 rate.
From there I simulated 1,000,000 games in which T1 is protecting a 1 goal lead for the remaining 2.5, 5, 7.5, 10, 12.5, 15, 17.5 and 20 minutes of a game under both normal style play and defensive shell style play. Here are the results at the end of regulation play.
Wins, losses, ties are T1’s record after 60 minutes and regulation win% is the standard regulation winning percentage using 2 points for a win, 0 points for a loss and 1 point for a tie. PlayoffWinRate is the winning percentage of T1 in a playoff game assuming that they would win 52.4% of all overtime games. OTL Pts% is the current regular season system where you get 1 point for an overtime loss, 2 points for a win of any kind and zero points for a regulation loss (under this system for simplicity sake I assumed a 50% chance of winning an overtime game since we don’t know odds of winning a shoot out).
That is a lot of numbers, so lets look at these in nicer easier to read charts.
Under this constructed scenario the break even point for when to go into a defensive shell and when to continue playing normal hockey is at about 7-7.5 minutes for regulation win % and playoff win % systems and about 13 minutes for the point for an overtime loss system currently used during the regular season.
For some people this may not make sense intuitively. How can it be better to stop playing a system in which you are expected to out score your opposition and start playing a system in which you are expected to score the same as your opponent. The reason is simple and it comes down to that over a short period of time your are essentially dealing with small sample size issues and randomness becomes more important than long term skill. The reality is, over a short time one team is almost as likely to score as the other so which team scored next is close to random, if any team scores at all. The most important thing when protecting a lead is simply reducing the likelihood that your opponent will score because the cost of your opponent scoring is far greater than the benefit if you scoring (it is irrelevant whether you win 3-1 or 2-1, a win is a win in the standings).
What is interesting is the effect of awarding the point for an overtime loss is in reality providing additional incentive for teams to play the defensive shell game for longer periods of time because the cost of giving up a goal is not as great in that system because a tied at the end of regulation guarantees you one point with the possibility of 2 where as in the other systems it does not. This means teams can play the defensive shell for twice as long as they could otherwise.
Of course, this is only looking at one side of the equation. Typically the trailing team will get more offensively aggressive even if it means increasing the possibility of having a goal scored against them. This is why teams pull their goalie late in the game. At that point scoring a goal is the only thing that matters so you may as well risk giving one up to score. Over the last 5-10 minutes or so it probably makes sense for the trailing team to take more high risk high reward plays in the offensive zone because at that point scoring a goal has more benefit than the cost of giving up a goal.