An Intro to Hockey Analytics/Statistics Part 2.1: The Importance of Context - Time on Ice

Take the offensive numbers of the following two players:

NAME Goals Assists Points
Player A
29 38 67
Player B
34 18 52

Player A looks more valuable than Player B right here. After all, while Player B would appear to be a better goal scorer, Player A has a ton more helpers. Surely he's the better offensive player, right?*

Well, maybe. See, hockey is NOT a sport where you can reach the full picture, or even a good part of the picture, by comparing players with a single statistic, in this case with points.**

**This isn't COMPLETELY true, there is a circumstance in which you can compare two players fairly well with one statistic. That would be when you compare goaltenders with Even Strength Save Percentage. We'll go over that later. But for the most part, what I say above is true.

Because there is one key fact of hockey that should not be overlooked:

Not All Ice Time is Equal

What does this mean?

Well, hockey players don't play 60 minutes, and the team's coach gets to try and choose which minutes each of his players will play. And certain minutes are easier to put up good numbers than others: it can be (relatively) easier for that player to score goals and tally assists if he is used in one way, while it might be harder for a player to put up such numbers if he is used in another manner.

So we can't simply just look at the results of the player on the ice to compare players....we need to factor in the context in which those players put up these numbers. How can we do that really? Can we really quantify context?

The answer to that question is yes, yes we can. Let's look at the ways we can do this:

Time On Ice
The first method by which we can attempt to quantify the context in which a player puts up numbers is actually quite simple: we look at the breakdown of the time on ice of the players in question. In how many minutes does each player put up these numbers? And Just-as-Important, in how much power play and shorthanded minutes does each player put up these numbers? As you should know instinctively, power play minutes are basically the easiest minutes to score on (especially 5 on 3s, but of course those are rares). By contrast, penalty-killing minutes are essentially the hardest minutes to score upon.
So let's look once again at Players A and B above, but this time look at their ice time.

NAME Games Played Even Strength ToI/60 Power Play ToI/60 Shorthanded ToI/60
Player A
79 15:34 3:30 0:10
Player B
76 12:38 0:54

Well this sheds some new light on these two players' offensive performances. Not only did Player A put up his numbers in three more games than Player B, he also got more minutes per game in order to put up such numbers. In fact, Player A has gotten over 300 minutes more ice time than Player B. This means incidentally, that Player B put up more points per minute played than Player A - but Player A simply got more ice time.

But more importantly, these Time on Ice numbers show us that Player A put up his numbers in quite easier ice time than Player B. Player A got a ton of power play time, the easiest time for putting up points - and Player A capitalized on this, with 24 points (9 goals, 15 assists) in this time. By contrast, Player B had much less power play minutes. and even strength minutes than Player A, but had a great deal more shorthanded minutes. In other words, while Player A got the bulk of his team's cushy minutes, Player B seems to have barely gotten any and in fact Player B has been burdened with some of the worst offensive minutes of his team.

Now Player A performed better than Player B in points per minute on the power play, in a small sample size. But that's not the key here: the key is that Player B played this harder ice time, and overall was more efficient at tallying up points.

So yes, Player A finished the year with 15 more points than Player B. But he did so in much more ice time, and easier ice time at that. Suddenly the gap between the two players' offensive numbers does not look so big, and in fact we might come off thinking that Player B is the better offensive player (It's not clear solely from Time on Ice that this is true, but it does make the point very credible). This is what we mean by taking CONTEXT into account. ==========================================
But simply looking at time on ice does not clearly tell us the whole story of course. After all, the vast majority of a player's ice time comes at even strength.

Several factors combine to make this so:
1. Some players play against tougher competition than others (Top line Defensemen, Defensive Forwards). 2. Some players are put on the ice for more faceoffs in the offensive zone than defensive zone, enabling them to start their shifts in a more favorable position for scoring and not being scored-upon (and vice-versa). 3. Some players play alongside better teammates, giving them greater support on offense and defense.

These are not the only 3 factors that can affect how hard or easy certain ice time is, but these are the three big factors (feel free to mention any factors I forgot in the comments - I can address these in the follow up posts).

Fortunately, we CAN, through the use of statistics, get a somewhat objective measure of all three of these factors and thus make an adjustment for context. In the next two parts of this series on context, we'll talk about each of these three factors in depth and how we can measure them. First up will be how we measure the difficulty of particular ice time (the first two factors)

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