DIAG-1 · Diagnosis · Invariant — holds by construction, no expiry
The delta bridge is closed
The law
For any KPI that is a sum or ratio of sums over a segmentation, the between-period delta decomposes exactly into definition-change + mix + within-segment rate + volume terms, once an interaction-allocation convention is fixed. Causes must act through these terms.
Scope
Additive numerators and denominators (spend, conversions, revenue, clicks and their ratios), not unique reach, medians, or deduplicated counts.
What this licenses you to do
Compute the bridge with a fixed weighting convention, confirm the terms sum to the observed delta, check the definition-change term first, and reject any diagnosis naming a cause outside the bridge.