Referrer

A referrer (also spelled "referer" in the HTTP specification) is the source that brought a visitor to your website. When someone clicks a link on Google, a tweet, or a partner's blog, the browser passes along the referring URL so your analytics can identify where the traffic came from.

Types of Referrers

Why Referrers Matter for CRO

Different traffic sources bring visitors with different intent levels, and understanding this helps you optimize:

• Organic search visitors often have high intent — they searched for something specific
• Social media visitors may be casually browsing — they need more convincing
• Referral visitors from a trusted source arrive with built-in credibility
• Direct visitors are often returning users who already know your brand

Knowing the referrer lets you tailor the experience. A visitor from a Google search for "best A/B testing tools" should land on a page that matches that query — not your generic homepage.

Referrers in Experimentation

Referrer data is valuable in A/B testing for:

• Segmenting results — A variant might win for organic traffic but lose for paid traffic. Breaking down results by referrer reveals these patterns.
• Targeting experiments — Show specific variants only to visitors from certain sources (e.g., a personalized landing page for LinkedIn ad traffic)
• Diagnosing anomalies — A sudden spike or drop in conversions might be explained by a shift in traffic sources, not by the variant itself

Referrer Limitations

• HTTPS to HTTP — Browsers strip the referrer when navigating from HTTPS to HTTP sites
• Privacy features — Some browsers and extensions block or truncate referrer data
• App traffic — Clicks from native apps (email clients, social apps) often arrive with no referrer
• Redirects — Intermediate redirects can lose the original referrer

UTM parameters (utm_source, utm_medium, utm_campaign) solve many of these issues by encoding the source directly in the URL rather than relying on the browser's referrer header.