DIAG-9 · Diagnosis · Invariant — holds by construction, no expiry
Definition changes break comparability
The law
Conversion counts produced under different definitions (events counted, attribution, dedupe) are different measurands; any comparison spanning a definition change confounds measurement with demand unless the change is modeled, bridged, or shown null.
Scope
Discrete changes (action added/removed) and gradual ones (modeled-conversion rollouts) alike. A bridged series, both definitions measured over an overlap window, restores comparability; that is the repair, not a counterexample.
What this licenses you to do
Enumerate every definition, attribution, and dedupe change inside any analysis window; insert a changepoint, mask, or bridge at each.