Save percentage (SV%) is a statistic that is commonly used to analyze goalies. Analysts have started using even strength save percentage (ESSV%) instead, for various reasons. One reason is that there isn't much data for special teams, so it's hard to draw any conclusions about performance or ability on the penalty kill.
Another is that a goalie doesn't control how many special teams situations he has to face, and so doesn't control how many special teams shots he has to face. If a goalie's team takes a lot of (non-coincidental) minor penalties, that goalie will face a lot of shots on the penalty kill. Those shots are tougher to stop, and this will tend to drag down the goalie's SV%. Even if he is one of the league's top goalies, his SV% may not reflect that because he is facing so many more short handed situations than the other top goalies in the league. ESSV% avoids all this.
There are still a couple issues with ESSV% though.
Problem 1: ESSV% still tends to be very inconsistent especially for goalies who have faced a relatively small number of shots. This has been noted by several analysts. The top three results in a google search for “save percentage regression to the mean hockey” are a good sample of articles that discuss these issues, and why ignoring these issues can lead an analyst to draw questionable conclusions about the ability of a goalie or about the other players on a goalie’s team.