Is Z-O Swing% a Better Indicator of Plate Discipline Than O-Swing%? by Dominikk85 September 24, 2017 In some FanGraphs articles, Z-O swing percentage is thrown around as a measure of plate discipline. That makes sense because generally when a hitter swings at strikes, good things happen, and if he swings at balls, bad things happen. To test if that stat is really better, I looked at the 2017 leaderboard. I looked at the wRC+ of the top 30 and bottom 30 hitters with Z-swing%, O-swing%, and Z-O swing%. Here is what I found: wRC+ z-o swing z swing o swing top30 122 112 122 bot30 103 105 96 all qualified 110 110 110 There is a slightly positive effect of Z-swing, but a much stronger effect of both Z-O swing% and O swing%. At the top, the low-chaser and high-differential guys do about the same, while the bottom chasers do even worse than the bottom differential guys. If you widen the search for top half and bottom half you get that picture: z-o swing z swing o swing 119 110 117 102 110 103 110 110 110 Z-swing has no effect at all, and the differential is slightly better than the chase rate, but not by much. Overall, the Pearson value for differential was a positive .42, for the chase rate it was .32 (used 100 – O-swing% to get positive value), and for Z-swing there was almost no effect (.07). So the differential is a bit better, but the effect isn’t huge; it is probably like with OPS+ and wRC+ where one is mathematically more elegant and correct but the actual values won’t differ much. I also dissected the hitting into the components OPB, ISO and BABIP. ISO z-o swing z swing o swing top30 .220 .210 .200 bot30 .170 .180 .163 all qualified .193 .193 .193 OBP z-o swing z swing o swing top30 .360 .335 .370 bot30 .324 .350 .317 all qualified .341 .341 .341 BABIP z-o swing z swing o swing top30 .308 .309 .306 bot30 .304 .303 .306 all qualified .306 .306 .306 The result is quite interesting. The differential (+27 ISO points) does clearly better in the power department than chase rate (+7); in fact, even Z-swing had a more positive effect (+17) on power than a low chase rate. With OBP, that is reversed. Here, the chase group does better than the differential group, while a high Z-swing rate has a negative effect. With BABIP there was a very small positive effect of differential and Z-swing, and no effect of the chase rate, but the effects are almost non-existent. So we seem to have two opposing effects here. Being more aggressive in the zone helps the power but seems to slightly hurt the OBP (of course there probably is a bias that aggressive hitters in the zone are often also aggressive outside, but still). And for OBP, chase rate clearly is king, while it doesn’t really have an effect on power. Still, that might have an effect for certain hitters and especially pitchers, but overall the advantage doesn’t seem to be big, even though it is a bit due to coincidence due to the opposing effects.