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6) The Codebook use case could start with the object "DataSet" and get all information needed from there.
7) From Barry: My thinking is that it makes sense to 'derive' or arrive at a variable using the path Ornulg and Wolfgang propose: the end-point, a Variable, contained in a Dataset, derived from a Capture (Question, Measure, etc.), which resides in a ImplementedInstrument, which is an instantiated ConceptualInstrument, which is the data collection plan of a StudyUnit. There seems a logical sequence in play here and thus one logical way to 'link' from higher level object like StudyUnit to a more granular level object like Variable. Instead of linking directly between the two, follow the natural progression or path between them.