Payment/checkout processes are probably the most important user workflows in your application, since they're the ones that most directly determine how much money you will make. A poor checkout process can kill an otherwise decent product's conversion rate. Christian Holst writes this researched post about how people perceive checkout processes:
One thing we noticed was that most people (8 out of 10 test subjects) imagine multi-step forms as a straight path with discernible steps. During a sign-up process, people would imagine the next page before actually seeing it - they would imagine a collection of pages, moving back and forth between them. Even though they hadn't reached the next step yet, they visualized it because they knew it was coming.
Even though processes aren't restricted to being linear in theory, from a UX point of view it helps to keep them so:
During our study, the checkouts that played along this mental model of a linear path with directional buttons, performed better than the checkouts that didn't play along.
From a technical viewpoint this whole idea of a linear path and direction is bogus. Sure we can enforce a certain sequence, but technically there's no concept of physical space - but as usual, this doesn't matter.
Creating an illusion of physical space, a path, is good usability and that's ultimately what matters.
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