Full Answer

The 30–40% figure is a composite of three independent blocking mechanisms that stack on top of each other. Each affects different tracking tools differently, making the total loss hard to measure from any single analytics platform.

Ad blockers are the most visible layer. Browser extensions like uBlock Origin and Adblock Plus block requests to known analytics and advertising domains. GA4's gtag.js, Facebook Pixel, TikTok Pixel, and Google Ads conversion tags are all on standard blocklist filters. When blocked, these scripts never load — no events fire, no cookies are set, the visitor does not exist in any marketing platform's data.

Browser privacy features add a second layer that operates independently of ad blockers. Safari's Intelligent Tracking Prevention restricts third-party cookies and caps first-party cookie lifetimes at 7 days for JavaScript-set cookies. Firefox Enhanced Tracking Protection blocks known trackers by default. These restrictions affect attribution windows and cross-session identification even for visitors without ad blockers installed.

Consent rejection is the third layer, specific to stores with European traffic. When a visitor denies cookie consent through the CMP banner, compliant implementations must block all non-essential tracking — producing the same data gap as an ad blocker but through a legal mechanism rather than a technical one. Server-side tracking bypasses the browser entirely for event delivery. When tracking fires on your server at the point of purchase or cart action, the event travels from your server to the analytics destination without passing through the browser where blockers and privacy features operate, recovering typically 20–30% of previously invisible events.