Wednesday, November 19, 2008

Shotblocking and Save Percentage

In a recent post by the Contrarian Goaltender,  he collectively analyzes data from 1999-00 to 2007-08 in order to determine the relationship between shot attempts against and save percentage    One of his findings was that there was a negative correlation between blocked shots and save percentage over this period.

To what extent,  however,  is this effect mediated by shot quality,  as measured by Alan Ryder?   On the one hand,  if the majority of blocked shots are those coming from the point and other peripheral areas of the offensive zone,  then the shots that "get through" would tend to be those from areas closer to the net.   One would expect this to effect to be reflected by the shot quality data.   On the other hand,  if blocking shots has the effect of interfering with the sightlines of the goaltender, then this effect would not necessarily be reflected in the shot quality data.   Both of these effects, if real, could account for the fact that teams that block more shots tend to have lower save percentages, on average.  However, without analyzing the date, it's unclear to what extent each process is operative.

I think that it goes without saying that both processes are,  to some degree,  at work here.   It would be unreasonable to suspect that the relationship between save percentage and shot blocking can be entirely accounted for by one single factor;  indeed, most causal relationships that exist in complex phenomena, hockey included, are multifactorial.   What I'd like to determine,  however,  is how much of the correlation can be accounted for by the shot quality data.

What I did was analyze the relationship between team blocked shots,  team save percentage,  team shot quality,  and team shot quality neutral save percentage for each season between 2002-03 and 2007-08.  I determined the correlations for each season individually because the total number of blocked shots is not uniform over time. Specifically, the pre-lockout values are significantly lower than the post-lockout values, and the 2002-03 values are much lower than that for 2003-04 (and every other season, for that matter).  Thus,  analyzing the data as a whole would preclude interpreting the results with any degree of confidence.   It is worth noting that 2002-03 is no arbitrary cutoff -- there is simply no data on shot quality prior to this season.   As is the case with the shot quality data,  there is a clear arena bias with respect to the recording of blocked shots.   This is evidenced by the fact that the standard deviation in home blocked shots is much higher than for that for road blocked shots.   The data:

What I did,  then ,  is incorporate the figures for both total blocked shots and road blocked shots.   Presumably,  road blocked shots would provide a better indication of the 'true' number of shots blocked as it less subject to arena bias.   Here are the inter-variable correlations:
It should be noted that the shot quality data for 2006-07 and 2007-08 is for road shot quality only -- thus,  I was unable to include the correlations between road shot quality neutral save percentage and road blocked shots for any season prior to 2006-07.   The correlations between road save percentage and road blocked shots were also excluded for these seasons.

Several points:

1.   There are slight,  yet consistently negative,  correlations between total blocked shots and save percentage.  Although I doubt that any of the specific correlations are statistically significant,  the fact that all of them are in the same direction is suggestive of an underlying relationship.    Also,  this accords with The Contrarian Goaltender's finding of a correlation of -0.30 between blocked shots and save percentage over a larger data sample.

2.  The correlations between total blocked shots and shot quality neutral save percentage show a similar trend in that for every season the relationship is mild yet inverse.  Thus,  shot blocking seems to have an effect on goaltender save percentage that is residual and cannot be accounted for by the shot quality data.  Of course, it's also clear that shot quality is also partly driving the relationship.    For one, there seems to be somewhat of a positive correlation between blocked shots and shot quality against.   Additionally, the fact that the relationship between blocked shots and SQN % is not as strong as the relationship between blocked shots and save percentage proper also shows how shot quality partially mediates the correlation.  If it did not, these two groups of correlations would be near identical.

3. The correlations for the 'road' data are generally weaker than the overall correlations,  which is counterintuitive as both road shot quality and road blocked shots are less distorted by bias.   This can probably be explained by the attenuated sample size for road games in each individual season (41 vs 82 games),  which diminishes the resolution of the data.    Indeed,  this also occurred in previous posts where the shot quality figures themselves were analyzed.