The past three years the Pirates have been a pretty good defensive team. The numbers and the eye-test generally matched up. There were plus defenders at short, catcher, center and left. Capable defenders at second and third and passable guys at first and in right. In addition the Pirates have been aggressive in adopting shifts and that has undoubtedly helped their ability to turn batted balls into outs.
Defensive efficiency isn't the perfect team metric. Loosely, it's the inverse of BABip. It measures how often a team's defense converts a ball in play into an out. It doesn't track double plays, assists or errors, but it's a simple way to see if a team is doing it's job in the field.
From Baseball Prospectus' glossary page explaining defensive efficiency:
The Defensive Efficiency spectrum based on the 2011 season:
Excellent - Tampa Bay .735 (1st)
Great - Texas .722
Average - Toronto .710
Poor - Pittsburgh .700
Horrendous - Minnesota .693 (30th)
Here is how the Pirates graded out the last three seasons:
2012: 10th .714 (range: .723 - .675)
2013: 5th .715 (range: .727 - .686)
2014: 11th .710 (range: .728 - .685)
The 2015 Pirates have been a different story. They are a terrible defensive team weighed down by their horrific infield defense. At first Pedro Alvarez has been a disaster. As I've mentioned before, Alvarez's inability to transition to first is the single most surprising thing about this Pirates season. Neil Walker is now a below-average defensive second baseman. He's solid at turning the double play but his range is poor. Jordy Mercer is a plus defender at short, but he's been out a month and Jung Ho Kang has been average in his stead. Kang is an excellent defensive third baseman, but most of the starts there have gone to Josh Harrison, surprisingly bad this season, and Aramis Ramirez, a disaster since coming over from the Cubs. It shows in the numbers:
2015: 25th .693 (range .722 - .678)
Last night in their 3-2 win over the Mets, the Pirates fielded a lineup of Alvarez at first, Walker at second, Kang at short and Ramirez at third. One thing I'm virtually certain of, none of those guys will be the starter at that position next year.