Baseball-reference labels Adam Ottavino as 0.1 IP (innings pitched).
Similarly, Jordan Montgomery gets 5.2 IP. What does that mean? How is this stat calculated?
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Pitchers IP or "innings pitched" stats are recorded as the number of outs that the defense was able to generate, listed in innings, during the time when the the player is the pitcher of record. Three outs is equivalent to one inning.
Any partial innings are shown as the number of whole innings, plus the number of outs after a decimal.
It is also common that if a pitcher starts an inning but has to leave before any outs are recorded, the inning count has a plus sign (+) next to it. So if Smith pitches the first four innings, then faces 3 batters in the fifth without recording an out before leaving, that line could be "IP: 4+"
In your example, the pitchers earned the following number of outs.
Although the decimal notation that is commonly used does not seem to be mentioned, the practice of recording outs as fractions of an inning is detailed in the MLB rulebook. 9.02(c)(1) lists that the number of innings pitched is an official stat to be recorded and has the following comment:
Rule 9.02(c)(1) Comment: In computing innings pitched, the Official Scorer shall count each putout as ⅓ of an inning. For example, if a starting pitcher is replaced with one out in the sixth inning, the Official Scorer shall credit that pitcher with 5⅓ innings. If a starting pitcher is replaced with none out in the sixth inning, the Official Scorer shall credit that pitcher with 5 innings and make the notation that that pitcher faced batters in the sixth, noting the number of batters faced. If a relief pitcher retires two batters and is replaced, the Official Scorer shall credit that pitcher with ⅔ of an inning pitched. If a relief pitcher enters a game and his team initiates a successful appeal play that results in one out, the officer scorer shall credit such relief pitcher with ⅓ of an inning pitched.