The Instance property of a Data-Field indicates how many occasions participants have measurements performed. There are three categories:
- Singular - only one instance can be present, for example sex or year-of-birth;
- Defined - more than one instance may be present, and each instance represents a fixed identifiable set of results across all participants;
- Variable - more than one instance may be present, however there is no correspondence between (say) the 3rd instance for one participant and the 3rd instance for another;
- If everyone was weighed on each birthday (and only on those occasions) then the data-field for weight would be a Defined Instance. If everyone was weighed on an ad-hoc basis, then it would be a Variable Instance.
- If all participants attended a clinic 3 times and had their height measured twice during each visit, then the data-field would have 3 instances, each of which being an array of 2 values.