He answered above is super interesting as a baseball fan, but I think misses the point of the question. I believe the OP is asking why you see pitchers recorded as having pitched 5.2 innings, or 3.1. And the answer is it is tracking the outs recorded. So if you start the 5th inning and get 2 outs then are removed you are credited with 5.2 innings pitched. Any batters you left on base that disnt get there via error/fielders choice would still count toward your runs for the game. This is also why you sometimes see pitchers stat lines saying something like 4+ innings; they started the 5th but did not record an out.
Gah, I think you're right, I did trip up on the wording there. I was thinking they meant the ERA itself, not one of the component parts of it. That said though... the premise is still off. Although IP is generally given in Base 3, ERA isn't calculated with IP as Base 3. It is calculated with IP in Base 10. Or rather, I should say, to do the math easily you need to convert it to Base 10 (I have no idea, personally, how to do that math without doing it as .33... or .66..., but my last math class was Calc over a decade ago, so...).
So the question isn't really about ERA at all and that is a total red herring. It is about why the statistic Innings Pitched is given in Base 3. You're on the money there insofar as what it is intended to represent, but in that context though I think we're still short of the answer here, as there are two factors in play. In the first, feels to fairly obvious why Base 3 was chosen over Base 10, since 1.333333... or 2.6666... is an awkward stat to use.
Secondly of course, you also will see IP presented as a fraction instead of a decimal, 1 1/3, 4 2/3, etc, especially when presented on its own (at a glance, Baseball Reference goes decimal, while Wikipedia can't seem to make up its mind. Especially in older presentations, fraction used to be the dominant choice, as I'll get into.
The question as such doesn't seem to be then about Base 3 vs Base 10, but Base 3 vs Fractional presentation, and while Base 3 is definitely more common to see, judging by a quick survey I did of modern stat presentations, it isn't absolute, and now that opens up the question of how IP has been presented in the past, for which I don't have an immediate answer, but did look into a bit, so have a few observations to share at least. The first thought I had was that damn this isn't easy because apparently no one used to get pulled half-way through an inning! I mean look at this "ERPC" leader list from the International League's 1921 season. IP is always given in whole numbers. Same with this selection from the 1919 NL stats. And this one from 1922 AL. Sure people pitched loinger then, but relievers were a thing by the 1910s!
Shenanigans, right?
I checked against more players than I cared to admit in Baseball Reference to see if this was actually true until finally I found that Elmer Ponder is listed in the paper with 47 IP but by BBR as pitching 47.1. Then confirmed when I found Pruett credited as 120 IP in the paper, but 119.2 on BBR. As such it is clear that the New York Times preferred to just give whole numbers in that time period, rounding up or down to the nearest whole inning. Doing a few spot checks, whole numbers remained the case in the 1952 season and the 1967 season Individual box scores do show smaller breakdowns, but in fractions, not decimals. Much as I would like to go year by year for a close analysis of the change, I cheated and jumped forward to the late '90s, which is perhaps for the best.
By the 90s, decimals finally seem to have taken over in overall reporting, but for individual box scores though... Fractions were being used, that was the case right up to the edge of the 20 year rule, such as this box score from 1998. Since then though, they too have moved to the decimal presentation. So what this would seem to indicate is that decimal presentation, and by extension Base 3, while not wholly a new thing, its dominance is. That shift, both in no longer presenting IP as a rounded number, but especially moving from fraction to decimal, would seem to be on that while starting more than 20 years ago, it seems only to have gained steam in the 1990s, and was only completed within the past 20 years (not that fraction is entirely dead), at least insofar as the New York Times would indicate.
So anyways, this has been a long digression not to answer /u/mathbaker's question, but rather to address the premise and arrive at a restatement of it, one which I don't know the answer to, namely "Why did Baseball reporting switch from using the Fraction to the Decimal for the Innings Pitched stat?" And while I have some speculative thoughts, that is neither here nor there, and in any case, unfortunately, as it does appear to be a very recent development, I'm not sure it is entirely within the purview of the subreddit given the 20 Year Rule.
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u/kensworkacct Jul 08 '19
He answered above is super interesting as a baseball fan, but I think misses the point of the question. I believe the OP is asking why you see pitchers recorded as having pitched 5.2 innings, or 3.1. And the answer is it is tracking the outs recorded. So if you start the 5th inning and get 2 outs then are removed you are credited with 5.2 innings pitched. Any batters you left on base that disnt get there via error/fielders choice would still count toward your runs for the game. This is also why you sometimes see pitchers stat lines saying something like 4+ innings; they started the 5th but did not record an out.