An e-commerce site's checkout conversion fell from 3.1% to 2.4% over two weeks. Desktop is unchanged at 4.0%. Mobile fell from 2.8% to 1.9%. Traffic mix is unchanged. Add-to-cart rate is unchanged on both. The drop is entirely between the payment page and order confirmation. A payment provider was added three weeks ago and now handles 45% of mobile transactions. Its published success rate is 94% against the incumbent's 97%. Mobile page load time rose from 2.1 to 3.8 seconds on the payment page in the same period.
Isolate the failure, list the candidate causes, use the figures to rule them in or out, and say what you would do next.
100 points, 60% to pass.
Two candidate causes arrive together, which is the point. The provider's 3-point success gap on 45% of transactions explains roughly 1.35 points of a 45-point relative fall — real but not the whole story. The load time nearly doubling on the payment page specifically is the larger suspect and would hit mobile far harder than desktop. Strong answers separate them with data that already exists: payment success rate by provider, and conversion by load-time bucket. Weak answers pick the new provider because it is the obvious change and stop. The recommendation should be able to act on both without waiting for a perfect attribution.