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UX Lessons from running more than 1,200 A/B Tests
Summary
Key Insights
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Nine out of ten AB tests at Booking.com fail, but the few that succeed compound into massive growth.
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Execution often fails on good ideas rather than the ideas themselves.
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Technical details like page load time can negate positive design changes if not carefully managed.
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Edge case bugs in different languages or currencies can silently kill conversions.
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Minor typography choices, such as switching from Times New Roman to Arial, have a large impact on conversion.
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Using system fonts improves legibility and page load speed, positively affecting conversions.
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Tracking should only include users exposed to the tested change to avoid noisy data.
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Qualitative user research drives many experiment ideas by revealing real user challenges.
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Guardrail metrics like customer service calls and loyalty prevent short-term wins from harming long-term value.
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Products should be retested over time as technology and user behaviors evolve, since what fails today may succeed tomorrow.
Notable Quotes
"I’m actually a really big failure — nine out of ten tests fail with no positive measurable impact."
"Compound effect means good upon good upon good eventually builds to incredibly fast growth."
"If a problem keeps coming up, it’s usually the execution of the idea that’s failing, not the concept."
"Increasing page load time by three seconds can nullify all your design improvements."
"Picking the wrong size image can impact customer experience as much as what you see on screen."
"There are as many versions of the website as stars in the sky because of language, currency, and device variations."
"No tracking is perfect because numbers tell you what likely happened, but not why it happened."
"System fonts load faster, improve legibility, and outperform fancy brand fonts in conversion."
"Guardrail metrics help catch unintended consequences like increased customer service calls despite higher conversions."
"You can’t fail 90 times out of 10 without laughing at yourself to keep going."
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