The invaluable J. Likens wrote on his blog a while back about how shot rate and shooting percentage depend on the score, finding, in particular that:

"The team that plays with the lead will tend to have a higher scoring chance/shot ratio than a team that plays from behind. This is because a team that plays from behind is forced to take more chances in an attempt to tie the score."

Now this always struck me as situation that would impact shot quality - in general, more scoring chances implies getting to the net, where shooting percentages are higher.  Previously, we've seen that shot quality plays some role in the shootout, that a top defensive coach can limit it somewhat, that it explains half of the increase in shooting percentage on rebounds, and that it shows up in the shooting percentages for defensemen relative to forwards.  So how does it impact even-strength shooting percentage relative to a tied game when a team is leading?

I had some really confounding results that I posted initially that I tried to explain away, but really couldn't.  I found that the database I'm working from, which comes straight from the NHL data feed, had some phantom score changes in it - it would be 2-1 one minute, 5-3 the next, and then back to 2-1.  So I reworked the database to fix that column.

The way to read this chart is as follows: a team that's up +1 goal expects a 4.0% increase in shooting percentage solely due to shot location.  In reality, they see a 21.8% increase in shooting percentage, which means the bulk of the difference is due to some 'other' factor - usually reduced defensive play by the trailing team that leads to better chances for the team that's up.  We also see that shot quality (the column labeled 'expected') goes up as a team gets a bigger and bigger lead. 

I'll post all the data on leaders and trailers tomorrow after I run a few more checks on the database...