Rested, recent, overworked. A reliever who threw yesterday is not the same arm as one who has had two days off, and a reliever who has thrown on three of the last four days is often unavailable whatever the box score says. Workload is read from pitch counts over the last three days and days since the last appearance.
Depth, not names. A bullpen with one excellent closer and nothing behind him is thinner than it looks the night that closer is down. Each team is scored on its top three arms weighted by availability, plus the rested arms on the bench that are good enough to matter (under 4.00 SIERA).
SIERA rather than ERA. ERA on fifty relief innings is mostly noise — it moves on defence, sequencing and luck. SIERA is built from strikeouts, walks and batted-ball type, which stabilise far faster. Arms without enough innings for a trustworthy SIERA are shown unrated instead of being given a number nobody should trust.
This page shows what the data says. What you do with it is your business — we never publish predictions.