Theory:
Presenting the same option in different formats (or 'frames') can alter people's decisions.
Application:
Research in 1981 (link) told us that people choose differently between the exact same options if the options are presented differently (take a look at the Framing effect article on Wikipedia and the examples shown there). Take a look at the way you present options to your users. What is the 'story' around each option and is the option you'd like people to choose framed in a way that it's most likely to be chosen? For example: if you like people to subscribe to something for a small periodical fee, it helps if you can place the amount in a frame that immediately makes sense to people like 'Subscribe now for less than a cup of thee a day'.
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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.