The hierarchical ontology is a DAG. It's got a root, it propagates from the root to leaf nodes, and there is no recursion.
A fully tag-based system is neither directed nor acyclic. My view is that the better ones appear as DAGs withiin a localised area, but that constraint falls at a broader level.
The main problem with tag-based systems is that they tend strongly to be "folksonomies", where terms are not controlled and there can be multiple variants of spelling, naming, etc., often reflecting both personal, geographic, and temporal preferences / ideosyncracies.
That's where a controlled vocabulary or defined set of relationships / associations (perhaps probabalistic) helps. E.g., "Yerba Buena" -> "San Francisco", "Frisco" -> "San Francisco".
The Library of Congress Classification and Subject Headings make for an interesting study. They're quite extensive and have evolved over more than a century. They have their own inconsistencies, ideosyncracies, and controversies. Part of their strength is the surrounding infrastructure establsihed to support and address these. There are of course other classifications.
How exactly is a tag-based system not a DAG, regardless of whether tags can imply other tags or not?
> It's got a root, it propagates from the root to leaf nodes, and there is no recursion.
A network of instances and tags satisfies the same properties. Do you have any counterexample? If not, I have to dismiss a notion that "tag-based systems" stand in opposition to "hierarchical systems" since every network of objects and tags is a DAG.
The hierarchical ontology is a DAG. It's got a root, it propagates from the root to leaf nodes, and there is no recursion.
A fully tag-based system is neither directed nor acyclic. My view is that the better ones appear as DAGs withiin a localised area, but that constraint falls at a broader level.
The main problem with tag-based systems is that they tend strongly to be "folksonomies", where terms are not controlled and there can be multiple variants of spelling, naming, etc., often reflecting both personal, geographic, and temporal preferences / ideosyncracies.
That's where a controlled vocabulary or defined set of relationships / associations (perhaps probabalistic) helps. E.g., "Yerba Buena" -> "San Francisco", "Frisco" -> "San Francisco".
The Library of Congress Classification and Subject Headings make for an interesting study. They're quite extensive and have evolved over more than a century. They have their own inconsistencies, ideosyncracies, and controversies. Part of their strength is the surrounding infrastructure establsihed to support and address these. There are of course other classifications.