Usability is marketing. If understood properly. This is said as someone who navigates ecommerce sites for a living.

Some important percentage of those visitors who never make it to the checkout screen were victims of poor usability. I wouldn't try to learn usability from Neilsen unless I were an academic, as a retailer I'd learn from the volume players: Victoria's Secret, Netflix, Dell, 1-800-Flowers, Drugstore.com, Amazon, etc.

Copy them, they know usability that pays.

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...the Rockies may tumble, Gibralter may crumble... G & I Gershwin, 1937