Comment:

Remove required attribute in Observation.related.target

Not crystal clear about whether the target is the variable that holds a calculation/derived value or the one that generates it. But in either case, I would not make them required. For 40+ years, labs have been reporting ratios of one chemical to another and the ratio is usually clear from the name of the derived test and for 10+years reporting BMIs. And no one has had ANY problem with it. Think it is good to have the option, but it should NOT be required. Furthermore, lots of things are really derived from others- but no one notices (e.g EIA indexes are all ratios of patient to control optical densities)-- so requiring it will induce various excesses depending on the eyes of the beholder.

Summary:

Observation.related.target