Myth: a unified ID or CDP will restore the cross-device attribution cookies gave us

Myth: a unified ID or CDP will restore the cross-device attribution cookies gave us

The question: with deterministic identity resolution — a customer data platform (CDP) or universal ID stitching logins across devices — can you rebuild the complete cross-device path that third-party cookies once provided?

What the coverage data shows: deterministic identity only resolves users who authenticate. Industry measurements of logged-in coverage for typical e-commerce and media sites land well below half of sessions; the rest stay anonymous or get probabilistically guessed. So your 'unified' view is unified only for your most engaged, logged-in segment — and that segment is systematically different from the population you're trying to measure.

The nuance: this creates a measurement-induced selection bias. The users you can fully stitch are heavier, higher-intent, more loyal. If you compute attribution on the resolved population and extrapolate to everyone, you over-credit the channels that loyal users favor (brand, email, direct) and under-credit top-of-funnel channels that touch the anonymous majority. The path data got cleaner for a biased subsample, which can make conclusions worse, not better, if you forget who's missing.

Bottom line for practitioners: a CDP is genuinely valuable for activation and for the logged-in cohort, but do not treat unified-ID paths as a representative restoration of pre-cookie attribution. Report resolution rate alongside every attribution number, weight or model the unresolved segment explicitly, and keep aggregate methods (MMM, geo-tests) for the population your ID graph cannot see.