Monday, December 1, 2008
Shot Quality Part Two: Shot Qualify For
In previous post, I analyzed the utility of shot quality against [hereafter SQA]. The purpose of this post is to explore the utility of its counterpart -- shot quality for [SQF hereafter]. Now, if the data for SQA is reliable and valid, then the data for SQF ought to be as well. It just wouldn't make sense otherwise. Nonetheless, I think that it's necessary to show that SQF is a legitimate construct in its own right, not only to further affirm the utility of shot quality as a whole, but to allow SQF to be identified as one of the main components of team offence.
The fact that shot quality is fraught with arena bias is no less relevant to SQF than it is to SQA. One way to get around this is to simply use the data on road shot quality. The problem is that, of the three seasons of data for SQF (2003-04, 2005-06, 2006-07), only one decomposes the information into home and road situations. Nonetheless, the 2006-07 data allows for estimations of the degree of bias present at each arena. These estimations of bias can then be used to adjust the 2003-04 and 2005-06 data so as to reduce the distortion. The result is what I've termed 'adjusted shot quality' -- a relatively bias-free estimation of the relative dangerousness of shots that a team directs at the opposition net.
Adjusted SQF is a clearly superior metric to unadjusted SQF. For one, it has more construct validity. If the shot quality data is actually measuring what it purports to, then there should be a positive correlation between SQF and shooting percentage at the team level. As the graph below shows, the correlation between save percentage and SQF is higher for the adjusted SQF data than it is for the unadjusted data.
But what about reliability? It goes without saying that the values for unadjusted SQF are going to be reliable on a year to year basis due to the element of arena bias. However, will the same be true of adjusted SQF? That is to say, once the effect of arena bias is removed, is SQF still a reliable phenomenon?
[In the event that the information that the graph is supposed to convey isn't apparent, the blue columns show the correlation between a team's SQF in 2005-06 and a team's SQF in 2006-07. The red columns show the same correlation, except for the years 2003-04 and 2005-06. The columns on the right are for unadjusted SQF; those on the left are for adjusted SQF.]
The answer is clearly yes. The interyear reliability for adjusted SQF is almost as high as that for unadjusted SQF. Thus, SQF is a substantially reliable metric, and this effect is largely independent of measurement bias.
Thus, just as SQA is a reliable and valid component of team defence, so too is SQF a valid and reliable component of team offence.
2003-04 shot quality data -- study done by Ken Krzywicki
2005-06 shot quality data -- study done by Ken Krzywicki
2006-07 shot quality data -- study done by Alan Ryder