DIAG-2 · Diagnosis · Invariant — holds by construction, no expiry
Concurrent changes destroy identifiability
The law
Multiple changes landing coincidently on one unit are point-identified only jointly: the unit's series yields one confounded contrast. Individual attribution requires external structure, temporal separation, an untouched control segment, randomization, or exclusion restrictions.
Scope
Includes platform-initiated changes (auto-applied recommendations, learning resets) in the change count. Distinct effect signatures (CPC vs CTR) are exclusion restrictions, name them as assumptions, not observations.
What this licenses you to do
Before attributing a shift to any single change, enumerate every change on that unit in the window, including platform-initiated ones, and if more than one landed, either find a control or report the joint effect.