Well, that's not entirely true. We have team defensive stats; we know how many goals each team allowed. And since Point Allocation uses a top-down approach, beginning with team results and then allocating these results to the players on that team, that's actually quite a good start. But there's no way we'll get anywhere without some judicious fudging. Remember that the system is not designed to identify who the best players are in any era; it largely assumes that those players considered to be the best were the best, and bends to make them so, to a reasonable degree. The system is meant to put players from different eras on the same relative footing, and to evaluate the players from all eras that are not among the very best. In a sense, the very best players in a particular league-season are used to calibrate the Point Allocation system for that particular league-season, via the league quality adjustment. The ratings of other players fall out of that.
Before we allocate defensive points to players, we first divide them between the team's goaltenders and its skaters. On a league-wide basis, this is done in order to make netminders (who receive no offensive points) as valuable, on average, as any other position. And then we have our first fudge factor. The reputation of a team's goaltender, versus that of its skaters, is considered when dividing defensive points among them. We can't assume a constant ratio of goalie to skaters points for every team in a league; that's just not realistic. And since we don't have anything like save percentage to evaluate goaltenders on something other than team goals-against average, we need this adjustment. Defensive Point Allocation is just full of fudgy goodness, and all the better for it.
From there, it's a simple matter to allocate points to goaltenders, if a team had more than one in a particular season. Each goaltender's Marginal Goals saved is calculated based on his GAA, and the points are allocated on that basis.
Now for the skaters, we first use the basis of the very first edition of Point Allocation from 2002 or so. That is, if we know how much a player played, and we know approximately how good he was offensively, we should be able to derive that player's approximate defensive ability, since offence plus defence should equal playing time. So when you have a player who produces a relatively low amount of offence (compared to his position, and his teammates), the system assumes he's probably a relatively good defensive player. It doesn't assume a perfect relationship between the two, of course, and sometimes this will result in some inaccurate results, but this is largely overcome by examining players by career (or at least groups of seasons) rather than by individual seasons.
The same logic is used for the amount of defence assigned to each position as a baseline. The standard defensive contribution of each position is determined based on the league-wide offensive contribution of each position, such that each position is approximately equal in overall value. On top of this, and the adjustment mentioned above, we have the other big 'ole fudge factor. Players with excellent defensive reputations have their defensive value adjusted upwards, and those with poor reputations are adjusted downwards. It's as much art as science, of course, but just doing straight computations on these numbers will produce less accurate results than if we apply what we know about these players, that is not necessarily reflected in their stats.
Oh, one final thing. Players lose points based on the penalties they take, since penalties cost your team goals. That's about it for the system. There are probably some details I've left out, which I'll mention when I think of them. Let's have a look at the complete results for the 1907 Montreal Wanderers, which we've been using for an example so far, normalized to an 80-game schedule, 20 minutes per game for forwards, 25 for defencemen and 60 for goaltenders. OP is Offensive Points, DP is Defensive Points, PP is Penalty Points, TPA is Total Points Allocated and TPAK is the same per 1,000 minutes.
1906/07 Montreal Wanderers
You can see the Wanderers had two absolute superstars in Russell and Stuart, a bunch more playing at an elite level, and two others (Blachford and Glass) producing solid if unspectacular results. Little wonder the team was undefeated in 10 games.