Wow, I’m really harping on this Miguel Cabrera at third base thing, aren’t I? I promise I’ll write about something else eventually. Maybe.

UZR/150* is a nice number, don’t get me wrong. It’s an easy way to take a career or a group of seasons and divide it out on a “per season” basis (UZR/150 is quite literally UZR per 150 games played at the position). But UZR/150 has a context that needs to be understood before we go around citing numbers.

*UZR stands for Ultimate Zone Rating, an ‘advanced’ defensive metric that estimates a player’s contribution to the ballclub (reported in runs above/below average).

You can find a comprehensive UZR Primer on FanGraphs if you want to learn more about the statistic, but an over simplification is this: the field is divided (sort of like a grid) into multiple ‘zones’, and each fielder is credited by how many plays he makes (or fails to make) in his zones. Other factors that are considered include batted ball type and speed of the runner (as well as a few other things). But some factors are hard to determine, like initial position of the fielder or exact batted ball speed. This is all to say that UZR is a very good metric (likely the best defensive metric out there), but it is subject to a lot of randomness.

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All statistics are subject to randomness. We all understand this (to some degree) with stats like batting average. One game tells us almost nothing about how well a player actually hits (Miguel Cabrera could go 0-4 and Don Kelly could go 3-3), but a full season gives us a pretty good idea (although there’s still a fair bit of randomness at work in a season if you think about it). The rule of thumb for UZR is that three full seasons of defensive data are about as reliable as one full season of batting data.

 

Another way to put it is that one full season of UZR (say, 150 games played) gives us the same indication of a player’s true defensive abilities as 200 plate appearances does for his offensive talent.

In 2008, Miguel Cabrera’s first season with the Tigers, he played 14 games at third base and recorded a -4.2 UZR (that’s 4.2 runs below average). UZR/150 takes that number and straight-line-projects it all the way out to -36.8 runs (if he played 150 games).

But we’re only talking about 14 games. Using the rule of thumb listed above, that would be similar to deciding his batting talent after only five games. We all know that’s absolutely crazy. What would a sampling of approximately 20 PA’s tell us about Miguel Cabrera? You tell me. After five games in that very same 2008 (covering 21 plate appearances), Cabrera’s slash line read .111/.238/.278. No one would put any stock into such a silly sample size.

We should no sooner believe that Cabby’s true defensive talent was -36.8 that year than we should believe that he had a true OPS talent of .516. The two numbers pretty much have the exact same reliability (which is to say they’re both totally unreliable). So it’s not even worth brining up.

Matt Snyder is the editor of The Tigers Den. He can be reached on Twitter @snyder_matthew.