The two proposed models seem to have very different scope and goals, they don't seem to be two alternative approaches to the same goal. I think this makes them challenging to compare.

It was mentioned during one of the earlier conversations that the multiple item model helps solve some long-standing challenges. What are those? Does the multiple holdings model not address them?

Which of these models do folks want tomorrow? Which do folks want in a few years time? Is the answer to both of these questions, the same or different?

I think it is important to note that these models effectively take inventory (and closely related areas like circulation) down two different paths. It is possible to combine those paths later, though that will have the complexity of both paths.

What important aspects have I missed from my summary of the models below?

Models

Multiple Holdings for an item

Emphasises that a bound-with item is a single unit of circulation and that the copies of the bibliographic entity within it are not separate.

Characteristics

• Items may represent either a single copy of a bibliographic entity or a set of them bound-with each other
• There is no way to describe information specific to each of the copies of the bibliographic entity
• The only way to identify this is to infer it from the cardinality of holdings related to the item
• Is a structural change to the relationship between items and holdings records
• Any user or process which interacts with items in any way will need to change

Questions

• Does this model fulfil the needs of analytics?
• Which holding should be used when determining effective location, call-number etc? If the system picked at random, would that be acceptable?
• Which instance should be used when determining bibliographic information? If the system picked at random, would that be acceptable?
• Can an item be associated with no holdings?

Implementation Characteristics

• Effort
• Risk
• Maintenance

Multiple items for a bound-with

Emphasises that the copies of bibliographic entities within a bound-with are separate, whilst trying to retain that they cannot be circulated independently (also includes other processes that items are needed for).

Characteristics

• All items represent an individual copy of a bibliographic entity
• Some (either independent or primary) items also represent a unit of circulation
• Introduces an interdependency between items
• Fulfils the needs of analytics
• Any user or process that either need to update or circulate items (and maybe other processes) need to be able to distinguish and understand the three different types of item e.g. they need to know a barcode of a dependent item cannot be changed directly
• Requires copying information between items within a bound-with to keep them consistent e.g. barcode, location etc
• Requires removing some the constraints on an item, e.g. barcodes can no longer be unique

Questions

• Which items can be requested, only the primary / independent, or any?
• Which items are checked out, is it only the primary or all?

Multiple items and single unit of circulation for a bound-with (Marc's more radical model)

Separates the ideas of copies of bibliographic entities and units of circulation. In effect, this model makes the idea that there isn't a one to one relationship between copies and units of circulation (that the multiple items model expresses) explicit.

Characteristics

• Items represent an individual copy of a bibliographic entity
• Items no longer represent a unit of circulation, these are modelled separately
• Units of circulation are made up of items
• Circulation processes would rely on the unit of circulation (they would still needs to understand items, holdings and instances to provide bibliographic information)
• Requires changing many of the circulation models (which might involve complex upgrade processes)
• Requires moving some of the attributes of an item to a unit of circulation

Questions

• Is it an item or a unit of circulation that has a status? This might be especially interesting for processes like cataloguing, where all of the copies in a unit of circulation would go to that process, yet not all might actually be catalogued.