What is an Inherited Runner? Definition, Formula, and Example
An inherited runner is a baserunner who reaches base against one pitcher but is charged to that pitcher even after a reliever enters and either strands or allows the runner to score.
Inherited Runner Definition
An inherited runner is any runner on base at the moment a relief pitcher enters the game. The runner "belongs" to the previous pitcher for earned run accounting — if the inherited runner eventually scores, that run is charged to the pitcher who allowed him to reach base, not to the reliever who was on the mound when he crossed the plate. This is the mechanism that keeps a reliever's ERA from being unfairly inflated (or, less often, unfairly protected) by the mess he inherits from someone else.
How Inherited Runners Are Tracked
Two companion stats track this: inherited runners (IR), the count of runners on base when a reliever enters, and inherited runners scored (IRS), how many of those runners eventually score. From these, analysts compute an inherited runners scored percentage:
IRS% = Inherited Runners Scored ÷ Inherited Runners
League-average IRS% sits around 30-32% — meaning the average reliever strands roughly two-thirds of the runners he inherits. Elite firemen who specialize in entering mid-inning with runners on — rather than starting a clean inning — are judged heavily on pushing this number well below 25%. The stat also feeds run-scoring-responsibility charts used in bullpen usage analysis, since a reliever who racks up a strong ERA but constantly hands his own bequeathed runners to the next man is inflating his own numbers at a teammate's expense — the mirror-image stat, "bequeathed runners," tracks exactly that.
Worked Example
Consider a reliever who enters 40 games with a runner already in scoring position and strands 32 of those 40 inherited runners. His IRS% is 8 ÷ 40 = 20%, comfortably better than league average and exactly the profile teams look for in a multi-inning fireman used in the highest-leverage jams — the role Andrew Miller filled for Cleveland during the 2016 postseason, regularly entering mid-inning with runners on to extinguish the Indians' biggest threats rather than starting innings clean. A pitcher who instead strands only 20 of 40 (IRS% = 50%) is functionally worse in exactly the situations he's asked to specialize in, even if his own runs-allowed ledger looks fine because those runners scored on someone else's line.
Why Inherited Runners Matter
Run prevention isn't cleanly divisible between pitchers in a single inning — a manager who brings in a reliever with runners on is betting that pitcher can finish the job his predecessor started. IRS% is one of the few stats that isolates that specific skill: entering under pressure, with runners already in scoring position, and getting outs before damage compounds. It's central to evaluating true bullpen "firemen" (as opposed to setup men who mostly start innings clean) and it exposes cases where a starter's ERA understates how often he left a bullpen holding the bag.
Limitations and Common Misconceptions
IRS% has small-sample volatility — a reliever who inherits only 15-20 runners in a season can swing from "elite" to "average" on a single bloop double, so it's most meaningful over multiple seasons or large workloads. It's also not park- or opponent-adjusted, and it says nothing about a pitcher's performance in his own clean innings — a great fireman can still have an unremarkable overall ERA if he's stuck pitching in front of a leaky defense.
In Legends Deck
Because Legends Deck's simulation charges runs to the pitcher who allowed the baserunner rather than whoever's on the mound when he scores, bullpen cards used as multi-inning firemen are rated in part on their real-world IRS%, rewarding pitchers who reliably strand inherited runners in the simulated bullpen logic.