Medication adherence is measured in a number of different ways (patient interview, pill count, refill timing, self-report of doses taken, etc). These methods have different weak points, and may disagree with each other, so multiple methods are often used to get a full picture of adherence. Different systems may also use different metrics depending on what data they collect:

Suppose a patient is taking their med every day at half the prescribed dose:

-Their payer may see adherence as a PDC score of 50% because the patient is getting fills half as often as they should be

-Their dose reminder app may see adherence as perfect because the patient taps off that they took the med every day

-The EHR used to conduct a patient interview may know that they're taking half a pill when they take the med. (The same EHR might have its own PDC score based on how often the patient is filling the med at the health system's pharmacy.)

 
Recommendation:
A field for the method used to measure adherence would be helpful to ensure clear communication between systems that may be using different measures of adherence. This could potentially be combined with informationSource (see FHIR-43466)