Base Rate Shift
Base rate shift is a change in how often an event occurs over time. It can break old baselines and cause calibration to drift.
Definition
A base rate shift happens when the underlying frequency of an outcome changes over time. That means historical base rates no longer match the current environment.
Why it matters
Many scoring and benchmarking methods assume stable base rates. If base rates shift, a baseline forecast based on old data can become misleading, and a previously calibrated forecaster can become miscalibrated.
How to handle it
• Use rolling baselines or recalibrate on recent data.
• Segment by time period.
• Track performance with rolling windows.
Related
Base rate shifts often accompany regime changes and affect relative metrics like Brier skill score.