[Crm-sig] Issue 336 and new Introduction & Scope

Martin Doerr martin at ics.forth.gr
Wed Mar 20 15:08:41 EET 2019

Dear All,

Here my attempts to reformulate the objectives of the CRM, its scope, 
and the methods of extensions. Please comment! To be discussed next week 
in the meeting.



  ISSUE 336


This document is the formal definition of the*CIDOC Conceptual Reference 
Model (“CRM”), *a formal ontology intended to facilitate the 
integration, mediation and interchange of heterogeneous cultural 
heritage information and similar information from other domains. The CRM 
is the culmination of more than two decades of standards development 
work by the International Committee for Documentation (CIDOC) of the 
International Council of Museums (ICOM). Work on the CRM itself began in 
1996 under the auspices of the ICOM-CIDOC Documentation Standards 
Working Group. Since 2000, development of the CRM has been officially 
delegated by ICOM-CIDOC to the CIDOC CRM Special Interest Group, which 
has been collaborating soon after with the ISO working group 
ISO/TC46/SC4/WG9 to bring the CRM to the form and status of an 
International Standard. This collaboration has resulted in ISO21127:2004 
and ISO21127:2014, and will be continued to produce the next update of 
the standard. This document belongs to the series of evolving versions 
of the formal definition of the**CRM, which serve the ISO working group 
as community draft for the standard. Eventual minor differences of the 
ISO standard text from the CIDOC version in semantics and notation that 
the ISO working group requires and implements are harmonized in the 
subsequent versions of the CIDOC version.

      Objectives of the CIDOC CRM

The primary role of the CRM is to enable the exchange and integration of 
information from heterogeneous sources for the reconstruction and 
interpretation of the past at a human scale, based on all kinds of 
material evidence, including texts, audiovisual material and even oral 
tradition. It starts from, but is not limited to, the needs of museum 
documentation and research based on museum holdings. It aims at 
providing the semantic definitions and clarifications needed to 
transform disparate, localised information sources into a coherent 
global resource, be it within a larger institution, in intranets or on 
the Internet, and to make it available for scholarly interpretation and 
scientific evaluation. Its perspective is supra-institutional and 
abstracted from any specific local context. This goal determines the 
constructs and level of detail of the CRM.

More specifically, it defines, in terms of a formal ontology, the 
*underlying semantics* of database *schemata* and *structured* documents 
used in the documentation of cultural heritage and scientific 
activities. In particular it defines the semantics related to the study 
of the past and current state of our world, as it is characteristic for 
museums, but also or other institutions and disciplines. It does *not* 
define any of the *terminology* appearing typically as data in the 
respective data structures; however it foresees the characteristic 
relationships for its use. It does *not* aim at proposing what cultural 
institutions *should* document. Rather it explains the logic of what 
they actually currently document, and thereby enables *semantic 

It intends to provide a model of the intellectual structure of the 
respective kinds of documentation in logical terms. As such, it is not 
optimised for implementation-specific storage and processing aspects. 
Implementations may lead to solutions where elements and links between 
relevant elements of our conceptualizations are no longer explicit in a 
database or other structured storage system. For instance, the birth 
event that connects elements such as father, mother, birth date, birth 
place may not appear in the database, in order to save storage space or 
response time of the system. The CRM allows us to explain how such 
apparently disparate entities are intellectually interconnected, and how 
the ability of the database to answer certain intellectual questions is 
affected by the omission of such elements and links.

  Scope of the CIDOC CRM

The overall scope of the CIDOC CRM can be summarised in simple terms as 
the curated, *factual knowledge* about the past at a human scale.

However, a more detailed and useful definition can be articulated by 
defining both the *Intended Scope*, a broad and maximally-inclusive 
definition of general application principles, and the Practical Scope, 
which is expressed by the overall scope of a growing reference set of 
specific, identifiable documentation standards and practices that the 
CRM aims to encompass, however restricted in its details to the 
limitations of the Intended Scope.

The reasons for this distinctions are twofold. Firstly, the CRM is 
developed in a “*bottom-up*” manner, starting from well-understood, 
actually and widely used concepts of domain experts, which are 
disambiguated and gradually generalized as more forms of encoding are 
encountered. This allows for avoiding the misadaptations and vagueness 
often found in introspection-driven attempts to find overarching 
concepts for such a wide scope, and provides stability to the 
generalizations found. Secondly, it is a means to identify and keep a 
focus on the concepts most needed by the communities working in the 
scope of the CRM and to maintain a well-defined agenda for its evolution.

The *Intended Scope* of the CRM may be defined as all information 
required for the exchange and integration of heterogeneous scientific 
and scholarly documentation about the past at a human scale and its 
evidence that has come upon us. This definition requires further 

  * The term “scientific and scholarly documentation” is intended to
    convey the requirement that the depth and quality of descriptive
    information that can be handled by the CRM should be sufficient for
    serious academic research. This does not mean that information
    intended for presentation to members of the general public is
    excluded, but rather that the CRM is intended to provide the level
    of detail and precision expected and required by museum
    professionals and researchers in the field.

  * As “evidence that has come upon us” are regarded all types of
    material collected and displayed by museums and related
    institutions, as defined by ICOM[1] <#_ftn1>, and other collections,
    in-situ objects, sites, monuments and intangible heritage relating
    to fields such as social history, ethnography, archaeology, fine and
    applied arts, natural history, history of sciences and technology.

  * The documentation includes the detailed description of individual
    items, in situ or within collections, groups of items and
    collections as a whole, as well as practices of intangible heritage.
    It pertains to their current state as well as to information about
    their past. The CRM is specifically intended to cover contextual
    information: the historical, geographical and theoretical background
    that gives cultural heritage collections much of their cultural
    significance and value.
  * The exchange of relevant information with libraries and archives,
    and the harmonisation of the CRM with their models, falls within the
    Intended Scope of the CRM.
  * Information required solely for the administration and management of
    cultural institutions, such as information relating to personnel,
    accounting, and visitor statistics, falls outside the Intended Scope
    of the CRM.

The Practical Scope[2] <#_ftn2> of the CRM is expressed in terms of the 
set of reference standards and de facto standards for documenting 
factual knowledge that have been used to guide and validate the CRM’s 
development and its further evolution. The CRM covers the same domain of 
discourse as the union of these reference standards; this means that for 
data correctly encoded according to these documentation formats there 
can be a CRM-compatible expression that conveys the same meaning.

      Coverage and Extensions

The intended scope of the CRM is a subset of the “real” world and is 
therefore potentially infinite. Further, the strategy to develop the 
model bottom-up from a practical scope has the consequence that the 
model will always miss some areas of relevant application or, on the 
other hand, some parts may not be developed in sufficient detail for a 
specialized field of study, such as /E30 Right/. Therefore, the CRM has 
been designed to be extensible by different mechanisms in order to 
achieve an optimal coverage of the intended scope without losing 
compatibility with the CRM.

Strict *compatibility of extensions* with the CRM means that data 
structured according to an extension must also remain valid as a CRM 
instance. In practical terms, this implies /query containment: /any 
queries based on CRM concepts should retrieve a result set that is 
correct according to the CRM’s semantics, regardless of whether the 
knowledge base is structured according to the CRM’s semantics alone, or 
according to the CRM plus compatible extensions. For example, a query 
such as “list all events” should recall 100% of the instances deemed to 
be events by the CRM, regardless of how they are classified by the 

A sufficient condition for the compatibility of an extension with the 
CRM is that CRM classes subsume all classes of the extension, and all 
properties of the extension are either *subsumed* by CRM properties, or 
are *part of a path* for which a CRM property is a shortcut. Obviously, 
such a condition can only be tested intellectually.

The mechanisms for extensions are:

 1. Existing classes and properties can be extended dynamically using
    thesauri and controlled vocabularies with CRM properties having as
    range /E55 Type/, as further elaborated in the section “About
    Types”. This approach is preferable when specializations of classes
    are independent from specializations of properties, and for local,
    non-standardized concepts.
 2. Existing classes and properties can be extended structurally by
    adding subclasses and subproperties respectively. This approach is
    particularly recommended to communities of practice needing
    well-established properties specific to classes that are not present
    in the CRM.
 3. Additional information that falls outside the semantics formally
    defined by the CRM can trivially be recorded as unstructured data
    using /E1 CRM Entity. P3 has note: E62 String/ to attach such
    information to the most adequate instance in the respective
    knowledge base. This approach is preferable when detailed, targeted
    queries are not expected; in general, only those concepts used for
    formal querying**need to be explicitly modelled.

      Conservative Extensions of Scope

Extensions may be incorporated in *new versions* of the CRM, or become 
*semi-independent modules* maintained in parallel to the CRM by 
communities of practice. In mechanisms 1 and 2 above, the CRM concepts 
subsume and thereby cover the extensions. This specialization as only 
method of extension would mean that the CRM from the beginning has 
foreseen all necessary high-level classes and properties. This comes in 
conflict with the very successful bottom-up methodology of evolution of 
the CRM itself and the development of extensions more peripheral to the 
current practical scope.

Extensions that are the result of widening the scope, rather than 
elaborating it in more detail, may quite well find a class “C” not 
covered by the CRM so far and even a superclass “B” of class C that must 
be regarded as a superclass of an existing CRM class “A”. From a 
logical-theoretical point of view, we precisely regard such extensions 
as compatible, if the CRM classes subsume all classes and all properties 
of the extension as long as instances are *restricted to the not 
extended scope* of the CRM.

In this case, an existing property p of class A may also hold for the 
new superclass B. We call the latter a *conservative extension*. That 
is, when restricted to the original class A, the extended property, p’, 
is identical to the original property p. In general, a superproperty is 
said to be a conservative extension of a subproperty when it is 
identical to the subproperty when restricted to its domain and range. In 
first order logic, the conservative extension of a property can be 
expressed as follows. Assume that A and C are subclasses of B and D 
respectively and  that p, p’ are properties between A,C and B, D 

A(x) ⊃B(x)
C(x) ⊃D(x)
P(x,y) ⊃A(x)
P(x,y) ⊃C(y)
P’(x,y) ⊃B(x)
P’(x,y) ⊃D(y)

If p’ is a conservative extension of p then

A(x) ∧C(y) ∧P’(x,y) ≡  P(x,y)

This is similar to what in logic is called a conservative extension of a 
theory. This construct is necessary for an effective modular management 
of ontologies, but is not possible with the current way RDF/OWL treats 
it. It has very important *practical consequences*:

1.Taken on its own, the CRM is not affected by such an 
conservative extension of scope, since it is not concerned with 
instances of class B that are not in class A.

2.If a conservative extension is incorporated into a *new version* of 
the CRM, the new version becomes *backwards compatible* with the 
previous one (therefore it is conservative).

3.The *bottom-up* development of ontologies encourages to find as domain 
and range of a property not the most general ones for all future, but 
the *best understood* ones, and leave it to conservative extensions to 
find more general ones in the future.

4.Extensions of the CRM maintained in separate modules that declare 
classes and/or properties not covered by superclasses and/or 
superproperties of the CRM *should* *clearly* *mark* the highest-level 
ones to be used by a respective query system in order to retrieve all 
instances described in terms of the CRM and the extension modules.

5.Extensions of the CRM maintained in separate modules must be 
harmonized with the CRM: All ontologically justified relationships of 
*subsumption* between the CRM and the extension should *explicitly* be 
declared and contained in the extension, or, if indicated, be submitted 
for the CRM to consider their inclusion.

It is the hope that over time the CRM and its compatible extension 
modules will provide a more and more complete coverage of the intended 
scope as a coherent logical and ontologically adequate theory of widest 
practical use. Besides others, this will require a collaboration of the 
involved communities based on a continuous effort of mutual 
understanding and respect.


[1] <#_ftnref1> The ICOM Statutes provide a definition of the term 
“museum” at http://icom.museum/statutes.html#2

[2] <#_ftnref2> The Practical Scope of the CIDOC CRM, including a list 
of the relevant museum documentation standards, is discussed in more 
detail on the CIDOC CRM website at http://cidoc.ics.forth.gr/scope.html

  Dr. Martin Doerr
  Honorary Head of the
  Center for Cultural Informatics
  Information Systems Laboratory
  Institute of Computer Science
  Foundation for Research and Technology - Hellas (FORTH)
  N.Plastira 100, Vassilika Vouton,
  GR70013 Heraklion,Crete,Greece
  Email: martin at ics.forth.gr
  Web-site: http://www.ics.forth.gr/isl

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