Why your LLM eval monitor keeps firing false alarms — and how to fix it

A software team running continuous LLM evaluations on production traffic found their quality monitor triggered alerts on four separate days in a single week, each time recovering without any fix. Statistical analysis reveals this was not a sign of real regressions: with 150 judge scores per hour and a true pass rate of 92 percent, there is a 53 percent daily chance of a false alert, making four alert days the most statistically likely weekly outcome. The core problem is that eval monitoring tools treat score thresholds as hypothesis tests but rarely ask users to specify window size, the key variable that determines false-alarm rates. Smaller sample windows require much lower thresholds to maintain the same statistical reliability, yet most teams set thresholds by intuition rather than calculation. Engineers are advised to calibrate alert thresholds using binomial statistics based on actual window size, or switch to count-based windows to keep false-alarm rates predictable and controlled.