Sticky add-to-cart bar with preselected size: +9.6% CR, inconclusive
Test run byClean CommitCRO agency
Commentary byLiftArchiveOur analysis, not theirsA sticky add-to-cart bar with a size dropdown and a preselected size showed a 9.6% conversion rate lift on mobile, but the result was not statistically significant, so we treat it as inconclusive.
Clean Commit ran this test for an anonymized Shopify store and published it in its A/B test library.
The targeted metric, add-to-cart rate, rose only 2.2%.
- Conversion rate: +9.6% and orders +8.3%, but neither reached significance.
- Add-to-cart rate: +2.2%, with only a 65.8% probability of beating the original. Inconclusive.
- Average order value: -6.9%, so revenue per visitor rose just 2.3%.
- Bounce rate: -10.4%, a sign shoppers stayed on the page longer.
The change
The variant adds a sticky bar with a size dropdown and a preselected size.
Simplified mockup drawn by LiftArchive from the shared screenshots.
Control
Mobile product pages had no sticky bar. Shoppers scrolled back to the main size picker and add-to-cart button.
Variant
A sticky bar stayed on screen with a size dropdown, a 'Find my size' link and a size preselected where possible, so the button was always active.
Why it likely came out this way
Our reading of the result, not the practitioner's claim.
- Keeping size and the buy button in reach removes a scroll back up, which may explain the lift in orders.
- A preselected size makes the button work on first tap, but some shoppers may add the wrong size, which can raise returns.
- The quick path may lead to single-item orders, which fits the drop in order value.
Caveats: Not statistically significant: add-to-cart had a 65.8% probability of beating the original. Sample size and duration were not shared, and order value fell 6.9%.
Who ran it
Clean Commit ran this test for a Shopify store (anonymized) and published the results (Clean Commit A/B test library). The numbers are theirs; the summary and commentary on this page are ours.
Read the original case study on Clean Commit →
How to run this test on your store
- Hypothesis
- If we add a sticky mobile bar with a size dropdown and a preselected size, then more shoppers will add to cart because they can buy without scrolling or picking first.
- Primary metric
- Mobile add-to-cart rate
- Guardrails
- Average order value
- Revenue per visitor
- Return rate for wrong size
- Who should test it
- Apparel and footwear stores with long mobile product pages. Be careful with size preselection if wrong-size returns are costly; you may prefer to require a choice.
Questions
Should I preselect a size on product pages?
It makes the add-to-cart button work on first tap.
The risk is shoppers ordering the wrong size, so track returns if you test it.
Is a 9.6% lift worth acting on if it's not significant?
Treat it as a signal to retest, not proof. Here the targeted add-to-cart metric barely moved.
Top or bottom sticky bar on mobile?
Both are common, and results vary by store.
Test the position against your own layout and any chat or cookie widgets.
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