If returning customers show up as new in your data and conversion attribution drops for journeys longer than a few days, you've hit a cookie lifetime limit. The main culprit is Safari ITP.
Symptoms
- A high share of "new" users even where you expect returning ones.
- Attribution works for short journeys but breaks down beyond about a week.
- The problem is pronounced especially on Safari and iOS.
Why it happens
Safari Intelligent Tracking Prevention (ITP) caps the lifetime of cookies set by JavaScript in the browser — i.e. classic client-side tracking — to 7 days, in some cases just 24 hours. Once the cookie expires you lose the visitor's identity and their next visit looks new.
How to extend the lifetime
- Deploy server-side tracking. The identification cookie is then set by the server via an HTTP header as first-party — such cookies aren't subject to the 7-day cap on JavaScript-set cookies and last longer.
- Turn on Cookie Keeper. It helps keep visitor identifiers consistent even in environments that aggressively clear cookies. Details in the docs: Cookie Keeper.
How to verify improvement
After deployment, watch the share of new vs. returning users and how long attribution stays effective. For longer buying journeys, the share of correctly attributed conversions should rise.
The full mechanism and its impact on conversions is covered in ITP, iOS and ad-blockers: lost conversions.