Saturday, 17 December 2011

Marginal Goals in History

One of the projects I've been working on for some time now is an application of my old Point Allocation system to hockey leagues throughout history. Point Allocation, developed in 2002, was the first published attempt at a comprehensive player valuation system, so far as I can tell. It's similar in purpose to Tom Awad's Goals Versus Threshold system used at Hockey Prospectus, in that it attempts to place a value on each player's contributions on both offence and defence in a single measure, to see who contributes the most to their team's success.

The basis of Point Allocation, then as now, is Marginal Goals analysis. This is an idea adapted from Bill James' work in his book Win Shares. Marginal Goals represent what a historically bad team is able to produce in competition; no team scores zero goals or allows an infinite number of them. Marginal Goals are therefore what you can accomplish without really trying to ice a good team. Historically, Marginal Goals on offence are equal to about 0.5 of the league average, and Marginal Goals of defence are 1.5 of the league average.

Any goals a team scores in excess of 0.5 times the league average therefore contributes to winning games, and any goals a team saves below 1.5 times the league average does the same. By adding these together, and dividing by twice the league average in goals scored, we get quite an accurate reflection of a team's winning percentage. As an example, let's look at the 1907 Eastern Canada Amateur Hockey Association (ECAHA) season:

TeamW-LGFGA
Montreal Wanderers 10-0 105 42
Ottawa Senators 7-3 76 54
Montreal Victorias 6-4 101 70
Montreal Winged Wheelers 3-7 58 83
Quebec Bulldogs 2-8 62 88
Montreal Shamrocks 2-8 52 117

Using Marginal Goals analysis, we get the following results, where MGF is the team's Marginal Goals For, MGS is the team's Marginal Goals Saved, and MG% is the resulting winning percentage:

TeamMGFMGSMG%
Montreal Wanderers 66.8 72.8 .913
Ottawa Senators 38.3 59.3 .645
Montreal Victorias 62.8 44.8 .704
Montreal Winged Wheelers 20.4 29.8 .334
Quebec Bulldogs 24.3 25.3 .329
Montreal Shamrocks 14.4 -4.2 .068

Teams from this time in history will tend to have a wider variance between their actual winning percentage and their MG% than modern teams do, because of the low number of games they played. With only a 10-game season, there is less chance for the luck to even out. But even so, the MG% tracks the teams' actual wins very well, with only the lowly Montreal Shamrocks being off by more than a win. The Shamrocks, a historically terrible defensive team, were lucky to win the two matches that they did in 1907, one against each of the Winged Wheelers and Bulldogs, two other rather bad teams.

The great advantage of this systems, of course, as compared to the more conventional Pythagorean Analysis, is that Marginal Goals gives us a very easy way to not only quantify a team's success, but allocate said success between offence and defence. In 1907, for instance, the Montreal Victorias relied on offence for 58% of their success, while Ottawa was 61% defence. This is the first step in allocating points to individual players, which is the purpose of Point Allocation.

Graphically, the relationship that Marginal Goals Analysis assumes between goal differential and winning looks like this:

As goal differential increases, winning percentage increases in a linear fashion. Now, you may be aware that this relationship isn't entirely realistic. Next time, I'll address that.

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