A subscriber gets the 'ebook bonus' pitch in your email, clicks through, and lands on a page testing the 'free trial' pitch. Two tools, two experiments, one confused user, and two datasets that can't be joined. This playbook covers keeping a single assignment authoritative across every surface.
This article is a stub.
The outline below is what the finished post will cover. Want it sooner? Tell us.
01How cross-channel contamination happens
- Independent per-tool randomization
- Why it shreds both experiments' validity, not just one
02The single-brain pattern
- One assignment service, every channel asks it
- ESP merge tags calling /assign at send time
- Landing pages resolving the same user to the same cohort
03Identity across channels
- Email address, anonymous web ID, and ad click IDs as one cluster
- What breaks when the links are missing, and degrading gracefully
04A worked example
- Newsletter x landing page campaign, end to end
- The outcome events each surface reports and how they join
Research brief
“How do teams keep experiment variant assignments consistent across email sends, landing pages, and ad campaigns? Cover deterministic hashing assignment, ESP merge-tag integration patterns, cross-channel contamination failure cases, and existing vendor approaches.”
The deep-research question this article will answer, sources cited in the finished piece.