Tuesday, August 24, 2010

Team Defense Costing Tigers Games in 2010

I think most readers of this blog understand the problems with using fielding percentage as a measure of individual or team defense, but I still see a lot of people using it in various places (e.g. television broadcasts and message boards).  So, I’ll review its limitations before moving on to better measures.  First, fielding percentage is the proportion of a team’s total chances (putouts, assists and errors) which result in either a putout or an assist.  The Tigers currently have a .982 fielding percentage which means they have avoided errors on 98.2% of their chances.  That ranks them 19th in the majors. 

One issue with fielding percentage is that error totals can be influenced by the subjectivity of official scorers.  An official scorer might occasionally give the home team’s fielders and hitters the benefit of the doubt by awarding hits on plays that might be called errors by another scorer.  Over the course of a season, this could influence a team’s fielding percentage.

Even if we assume that there is no scorer bias and that all scorers judge plays the same way, fielding percentage is still fundamentally flawed.  The problem is that it only penalizes fielders for errors made and does not charge them for balls that they can not reach.  It tells us nothing about the amount of ground covered by players and does not consider the difficulty of plays made or not made.

When Bill James introduced the defensive efficiency ratio (DER) statistic in the 1978 Baseball Abstract, it was the first time team defense had been formerly quantified in terms of range instead of errors.  DER is the proportion of batted balls in play, not including home runs, which are converted to outs by a team’s fielders.  For example, the Tigers have a .688 DER this year which means they have turned 68.8% of balls in play into outs. 

The Tigers are 19th in the majors in DER which matches the fielding percentage rank.  However, some teams rank quite differently on the two statistics.  The Oakland Athletics, for example, are first in DER, but only 13th in fielding percentage.  On the other hand, The Astros rank 29th in DER but 14th in fielding percentage.  So, the Athletics are a much better fielding team than the Astros, but fielding percentage makes them look like equals.

DER is limited because it does not consider factors such as types of batted balls allowed by the team’s pitchers (e.g. ground balls, fly balls), location of batted balls, how hard the ball was hit, handedness of pitcher and batter, and home ballpark.  The Ultimate Zone Rating (UZR) statistic introduced by Mitchel Lichtman in 2003 further refines the measurement of fielding by considering all those factors.  The Tigers have a UZR of +8.5 which means they have saved 8.5 runs compared to what you’d expect from the average team.  

The Tigers rank 10th in MLB in UZR, but that’s a little deceptive as UZR does not include pitchers and the the Tigers pitchers have not fielded very well this year.  A similar statistic which does include pitchers is Defensive Runs Saved or DRS (John Dewan, The Fielding Bible – Volume II).  The Tigers have a DRS of +2 with places them 19th in the majors.  The Tigers pitchers have a combined DRS of –14 which means they cost their team 14 runs versus what you’d expect from an average team.  Poor fielding by pitchers is why the Tigers ranks worse on DRS than UZR.

The Tigers ranked 9th in DER, 8th in UZR and 7th in DRS in 2009.  So, their fielding is clearly not as good this year as it was last year.  Their DRS in 2009 was +40, so, according to that measure, Tigers fielders have cost their team 38 more runs this year over last year.  Since, 10 extra runs is worth approximately one win, we can say that their defense is four wins worse this year. 

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