Recalling things from memory is hard. Recognition from previous experiences is much easier.
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.