Vocabulary: http://www.ivoa.net/rdf/datalink/core Author: François Bonnarel, Markus Demleitner, msdemlei@ari.uni-heidelberg.de Date: 2019-12-06 Superceded-by: VEP-004 New Term: sibling Action: Addition Label: Sibling Data Description: Data products derived from the same progenitor as #this. This could be a lightcure for an object catalog derived from repeated observations, the dataset processed using a different pipeline, or the like. Used-in: http://dc.g-vo.org/gaia/q2/tsdl/dlmeta?ID=ivo://org.gavo.dc/~?gaia/q2/199286482883072/BP This is GAVO's rendition of the Gaia DR2 epoch photometry, where users retrieve a time series in a specific band; the time series in the other bands are the siblings of that. Rationale: It is fairly common in complex pipelines that multiple data products result from a single observation. Often, this is true even in a single pipeline step, and hence the data products are not in a progenitor-derivation relationship. Still, researchers will want to know about these data products; for instance, while exploring a source in Gaia, a quick way to access epoch photometry or the RP/BP spectra is obviously valuable; such artefacts are not really progenitors of the catalog entry, though. In such cases, #sibling (or perhaps one of its future child terms) should be used. Clients should offer #sibling links in a context of scientific exploitation of the dataset (as opposed to, say, debugging). Discussion: In the discussion, it was the need for the concept as such ("other things that were produced from the observations that led up to #this") was not disputed, though the discussion was somewhat delayed by an investigation of possible shortcomings in the datalink data model (http://mail.ivoa.net/pipermail/dal/2019-December/008248.html) and whether additional cases should or should not be included in it (http://mail.ivoa.net/pipermail/dal/2020-February/008262.html). However, the main points of contention were the choice of the term and label ("sibling"). Objections included that astronomers might not understand the provenance-inspired nomenclature, that a very rough view of provenance must be adopted to actually talk about siblings (because, really, #this and the #sibling items just share common ancestors, not (necessarily) the parents), or that it is confusing to define, say, a spectrum to be the sibling of a catalogue row (http://mail.ivoa.net/pipermail/semantics/2020-May/002700.html). Possible alternatives investigated include #see-also (which was rejected as being too general), #co-generated (which was disliked because the implication that the two artefacts were built at the same time by the same processing step is even stronger than with #sibling), and #coderived (which wide acceptance but was strongly rejected by one party arguing it would strongly distort the meaning of "derived". In the end, #sibling was accepted as being acceptable after a splinter discussion during the May 2020 Virtual Interop, but the term #cogenerated was considered preferable, mainly because it better showed that the concept applies to elements in long provenance trees even if they are only related through distant ancestors.