Measuring True Pitching PerformanceThe TPP as a "Reverse TOP"(To understand this discussion, you need to already be familiar with the TOP--what it signifies and whence it comes. If you aren't, please first read the "White Paper" here on baseball analysis theory.)There are two "quality of pitching measures": the TPP and the QoP. The TPP is, in effect, the TOP of the composite of all the batters that a given pitcher has faced. That is only an approximate statement, meant to give you an idea of the concept, and we'll spell out the differences in a moment; but on that simplified basis, the TPP as a number signifies the number of runs that would be given up in a normal, full-length season by a pitching staff each member of which was pitching exactly like the man in question (assuming an average defense behind him). Also simplified, the QoP is the TPP translated into an ERA-like figure, based again on average defense and the normal percentage of total runs that are "earned" (which is a relatively stable percentage, about 90% in round numbers). Now let's look at the small differences. When calculating TOPs for batters, HBH somewhat idealizes the actual data. For one thing, we remove data for sacrifice bunts because SHs are (1) 100% managerially controlled, and (2) always operating to reduce overall run scoring; so it would be meaningless folly to include them in evaluating any man's ability to contribute to a team's offense. For another thing, we set stolen-base attempts (both SB and CS) to zero; that we do because at typical major-league levels of success--roughly 60% to 80%--the impact of stolen bases is thoroughly negligible, and even at more extreme removes from normal is just not material. (We do not analyze tactics yet on this site, but read up in any responsible book on analysis; base stealing is simply not a meaningful factor in run scoring in the large perspective.) For yet another thing, we use overall average values for sacrifice-fly numbers, since SFs are opportunity-dependent and do not vary enough to matter between one man and another. The TOP thus represents a man's ability to contribute to a team's run scoring under ideal conditions: no folly on his manager's or his part. When we analyze pitchers, the case is somewhat different. We need to use the full data set, because things like being bunted on, giving up sac flys, allowing or choking off stolen bases, and hitting batters are meaningful parts of a pitcher's game, and valid factors in any analysis of his performance and ability. Conversely, intentional bases on balls are--like SHs for batters--both 100% managerial and bad tactics. Thus, in constructing a true TPP, HBH uses only unintentional walk values (all walks minus IBBs). The Quality-of-Pitching Measure: the QoPThe QoP--Quality of Pitching--figure is just the TPP calculation, but with IBBs left in, so we get a number representing what the pitcher actually did, even though it is therefore not quite as good a measure of his actual performance as the TPP. But we generate the QoP so that folk used to ERAs can have something to look at that looks like numbers they're familiar with. The QoP still tells an interesting and vital story that the ERA does not, and that crucial difference is elaborated a litttle farther down this page, after the Table below.To help put these data, including the TPPs and QoPs you'll be seeing, in perspective, here are the numbers for the two Leagues and MLB as a whole during the 1977-1992 sixteen-year period that we use as a baseline in normalizing all data. The "ERA" value at the rightmost end is the actual ERA value.
(The very slight differences--under 1%--between the QoPs and the ERAs come from just using an average rate for estimating earned runs from total runs yielded.) Important Points To ConsiderERA/QoP CorrespondenceThis caveat has to do with data significance. In almost all cases, the ERA will correspond in a very broad way to the QoP; you won't find many 2.03/7.78 type pairings. But it is absolutely critical that you understand this: the ERA will not equal the QoP except by chance, nor should it. They are "measures" of the same thing, but signify very differently. The QoP tells just what its name says--the quality level of the man's pitching. Given a sufficient length of time pitching at that same quality level, the man will inevitably come to have a closely matching ERA; but that "sufficient length" may well be more than a single season--especially for relievers, far more. Again: the QoP is the actual quality of the pitching; the ERA is the result, with a lot of luck mixed into it.If you have read the background material on analysis theory, you will remember that this is all probabilistic work. What the QoP measures is the man's demonstrated norm of pitching behavior; what the ERA measures is the chance-influenced actual results of applying that behavior in games. Half heads and half tails is the norm of coin-tossing behavior; what we get when we actually toss a coin a number of times may in fact be quite different--but as that number of tosses increases, that difference will invariably become progressively smaller. So with ERAs and QoPs. If you want a crude rule of thumb, the expected average error in ERA versus QoP should be, in runs, about 10 divided by the square root of the BFP (batters faced) value. So if we have, let's say, a dozen pitchers who have each so far this year faced only 25 men, we would expect that the average difference between their QoP and ERA values would be 2 full runs! (Like a QoP of 3.00 and an ERA of 5.00.) And in several cases it would be more, occasionally much more. By the time our set of pitchers have faced 100 men each, the expected average error drops to 1 run--still a lot. If they are all starters, they might end up facing 900 men each over the season; then, the expected average error would be down to around 0.33 run. It should now be clear why it often takes several seasons for the ERA to really show what it is purported to show--that which the QoP shows at once, the quality of the pitching performance. Another factor, which we regret that we didn't realize in early years, is that a poor manager can artificially expand the QoP/ERA differential (as he artificially raises the pitcher's ERA). Consider: all probabilistic analysis, including our baseball work, relies on the data being independent and of equal value: any one coin toss is as likely to produce a head as is any other. Given enough data samples, the random peaks and valleys will average each other out. For batters, that is essentially true (actually, there are complicating factors which are beyond the scope of this discussion, but it's mostly true). For pitchers, it may not be so: a pitcher, unlike a batter, is performing in a continuous-effort mode, and a starter's work in the 8th inning is by no means necessarily the same as in the 2nd inning. A manager whose only idea of pitching use is "run 'em out there until they're in trouble" is a terrible manager, but--unfortunately--not a rare phenomenon. All too many pitchers are badly hurt by managers who insist on going to the well too often. A man who pitches splendidly through 6 and then decently but manifestly with effort through the 7th should be pulled then and there; but a manager who sends him out to get shelled in the 8th ("well, he looked pretty good up until then"--film at 11) is going to generate a data sample that is decidedly not representative of the man's work. His overall hits per batter faced, for example, may look good--but, because many of those hits were clustered in the few innings when he was tired and shouldn't have been out there--his actual ERA will be notably higher than what we calculate as his QofP. That is, he does pitch well on average, but his ERA is pumped up by his having been used when he wasn't at his average. In a civilized world, managers who work like that would be taken out back of the stadium, given a final smoke, and shot; but if we really did that, there would be very few still left to manage. The key point here--and it is the power and importance of the TPP and QoP--is that is is they, not the ERA, that tell you accurately how well the man is truly pitching. If his ERA is better, or worse, that's just short-term luck. So, with the understanding that there is some minor fuzziness owing to not using a fully exact stat set, the QoP is the value that tells you how well or poorly your chosen subject is really pitching. |
Measures calculated by High Boskage House Baseball Operations, using proprietary techniques.
All data soon will be (but is not yet) normalized for park effects and seasonal variations.
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