EvidenceVariable allows us to define eligibility criteria in a very expressive way due to  powerful features not covered in group:

Among features not covered in group:

• Combinations of characteristics ("any of", "at least 5", etc.) other than "all of"
• Modifiers to express how the characteristic is measured or determined
• Modifiers to express timing relative to an event*
• Defining characteristics with an expression language (e.g. CQL)*

However, the only way to reference EvidienceVariable presently is via Group:

Group.characteristic.code.text = "Eligibility Criteria"
Group.characteristic.valueReference == Reference(EvidenceVariable)

 This approach is not intuitive and has been causing confusion.

Allowing Reference(EvidenceVariable) directly from ResearchStudy would make usage much more intuitive.