Do good teams Create Good Luck? (Updated)
(Updated to include 3 seasons of data as I now realize that more luck data was available)
The other day there was a post on the Behind the Net Blog which used betting odds to estimate how lucky a team was during the 2009-10 season. In many ways it is quite an ingenious way to evaluate a teams luck and I recommend those who have not read it go take a look. Last night I was watching, sadly, the Leafs-Oilers game and thinking about luck in a hockey game and whether a team has any control over the luck they experience. It got me thinking, does a team which controls the flow of the play mean that team is more likely to have more ‘good luck’ stuff happen to them than ‘bad luck’ stuff.
I defined luck as being how many standard deviations their actual point totals were from their expected point totals as defined in the document referenced in the Behind the Net blog post and in an updated document with 4 years of data. I have only included 3 seasons in this analysis since I have only been working with 3 seasons of data recently and I was too lazy to go back and calculate a fourth season right now.
The most used stat to indicate how well a team controls the play is corsi or fenwick percentage which is basically the number of shots a team directs at the goal divided by the number of shots that they and their opponents teams directed at the goal. I’ll be using Fenwick % here which includes shots and missed shots but not blocked shots. So how does Fenwick % correlate with luck?
The correlation is fairly low but a correlation exists. Maybe good teams can generate their own luck. Here is a table of a teams luck and fenwick% for 2009-10.
|Detroit Red Wings||0.395||0.541|
|Toronto Maple Leafs||-1.282||0.528|
|New Jersey Devils||0.459||0.522|
|St. Louis Blues||0.186||0.519|
|San Jose Sharks||1.020||0.512|
|Los Angeles Kings||1.040||0.498|
|New York Rangers||-0.753||0.495|
|New York Islanders||-0.201||0.490|
|Columbus Blue Jackets||-0.855||0.488|
|Tampa Bay Lightning||-0.604||0.466|
When I was looking through the table something caught my attention. Of the bottom 15 teams in Fenwick%, only four teams had positive luck. These were Buffalo, Vancouver, Montreal and Colorado. Generally speaking, these four teams had good to very good goaltending. Of the top 15 teams in Fenwick%, only five teams had negative luck. These were Boston, Pittsburgh, Toronto, Calgary and Philadelphia. Boston and Calgary had good to very good goaltending (especially once Boston switched mostly to Rask) but Philadelphia, Pittsburgh and Toronto had mediocre to poor goaltending. That got me to wondering whether goaltending correlated with luck at all so I took a look at the correlation between 5v5 game tied shooting and save percentages with luck.
Like fenwick%, there is an indication of a small correlation between shooting percentage and luck and there is a bit more of a correlation with save percentage. Next I looked at combining all three factors. Initially I was going to look at combining all three through some sort of average but then decided to look at goals for percentage instead (goals for divided by goals for plus goals against) since that basically encompasses everything anyway and we find that combined we get a relatively strong correlation with luck.
Now we are getting into correlation that might actually mean something, but what does it all mean? To be honest, I am not sure. Regardless of what ‘skill’ we look at there does seem to be a small positive correlation between how good a team is and how good their luck is (as calculated from the betting lines). Does this mean that a bad team and especially a team with bad goaltending opens itself up to more bad luck than good teams or teams with good goaltending, or does it mean that luck manifests itself mostly in bad goals against or does it simply mean that the people who bet on hockey games trend towards betting the underdog which would push their expected winning percentage up and good teams expected winning percentage down which would result in a poor estimation of luck? I am not sure how you determine what the exact cause of the correlation is but if it is the latter I have a word of advice, always bet the favourite.