Theory:
Recalling things from memory is hard. Recognition from previous experiences is much easier.
Application:
When asking people to interact with the site, suggest (defined or popular) choices instead of letting them recall information from memory. Clicking fixed choices is much easier than filling in a textfield.
Recently I've seen some (often absolute) statements going around, generally in the line of "open source commerce platforms are a terrible idea". Now of course different solutions always have different pros and cons.
A hierarchy of evidence (or levels of evidence) is a heuristic used to rank the relative strength of results obtained from scientific research. I've created a version of this chart/pyramid applied to CRO which you can see below. It contains the options we have as optimizers and tools and methods we often use to gather data.