Metrics
Conversion Rate
The percentage of visitors or ad clickers who complete a desired action (purchase, signup, lead form). Calculated as conversions divided by clicks or sessions times 100. Ecommerce average is 2-3% but varies wildly by traffic source (3-5% for branded search, 0.5-1.5% for cold social traffic), device (desktop converts 2x higher than mobile for most categories), and price point (sub-$50 products convert 2-3x higher than $200+ products).
Why it matters
It is where ad performance and site performance become inseparable. A conversion-rate problem read as a traffic-quality problem sends budget away from a channel that was working.
In practice
Compare by landing page and by device before blaming the channel. A mobile rate half the desktop rate is a site issue, not an audience one.
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