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One of the key concepts in digital preservation is that of Constraint that defines limitations or restrictions on the space of allowable preservation actions. Constraints make the stakeholder’s values explicit and influence the digital preservation process. Constraints are often expressed

in Policies, and refer to Characteristics of PreservationObjects,

PreservationActions or Environments.

2.3.1 Significance constraints

The most common form of Constraints discussed in the digital preservation literature is

SignificanceConstraints, also called significant properties (for example, Hockx-Yu and Knight, 2008; Knight, 2008; Knight, Pennock, 2009), significant characteristics (Thaller et al., 2008; Becker et al., 2008a), essence (NAA, 2008), aspects (Clausen, 2007), and others. Original work on

SignificanceConstraints comes out of the Cedars project (CEDARS, nd), work at the Australian National Archives (NAA, 2008), the InSPECT project (Knight, 2008), Planets (Becker et al., 2008a; Becker et al., 2008b; Clausen, 2007; Dappert, 2009; TNA, nd) and others. Comprehensive surveys of related work in this area are provided by Knight (Knight, Pennock, 2009) and Wilson (2007).

They specify, as business Constraints, “the characteristics of digital objects that must be preserved over time in order to ensure the continued accessibility, usability, and meaning of the objects, and their capacity to be accepted as evidence of what they purport to record.” (Wilson, 2007). Section 3.2.3.2.2 discusses them in depth.

The term “characteristics”, which describes what must be preserved in this definition, is interpreted in two conflicting ways. Some interpret it to refer to the abstract properties of file formats (e.g., Becker et al., 2008a; Knight, 2008), whereas others interpret it to refer to the values of properties of specific digital objects (Becker et al., 2008b). One also finds different interpretations of the term “digital objects”, which describes which characteristics need to be preserved. In 2002, the OCLC/RLG Working Group on Preservation Metadata (OCLC/RLG Working Group on Preservation Metadata, 2002) stated that the properties of data objects need to be preserved; Brown (Brown, 2008) applies it to information objects as opposed to data objects in the OAIS sense of the terms (CCSDS, 2002); Becker (Becker et al., 2008a) applies it to the characteristics of specific file formats. Knight hints that the characteristics of the environments in which digital objects are rendered may also have to be preserved (Knight, 2008), but this idea is

not fully articulated there and is not developed until expressed by Dappert and Farquhar (2009b) and Anderson et al. (2010). Chris Rusbridge (Rusbridge, 2006) eloquently states why the quest for faithfulness to the original in all respects is both excessive and impractical in most preservation situations. The need to clarify the difference between SignificanceConstraints and representation information has repeatedly been voiced (e.g., Hockx-Yu and Knight, 2008; Knight, Pennock, 2009), but not previously addressed.

Relationship to DePICT

All previous treatments on SignificanceConstraints limit themselves to the identification of

Properties they consider significant either for certain file types or content types. This means that there are no allowances for tolerances, for specifying the relative importance of different

Constraints, for specifying pre- or post-conditions, for specifying Characteristics in the relationship between PreservationObjects rather than simply on one

PreservationObject or for specifying SignificanceConstraints on Environments. DePICT defines Constraints based on the requirements identified in the analysis of the digital preservation domain, addresses the gaps identified in the analysis, clearly defines the terminology and relates them to the DePICT conceptual model to avoid vagueness.

2.3.2 Preservation planning constraints

In a practical application, the Plato tool (Becker et al., 2008b) uses SignificanceConstraints

of PreservationObjects together with other Constraints to guide the preservation planning process. The Plato planning tool is a “decision support tool that implements a solid preservation planning process and integrates services for content characterisation, preservation action and automatic object comparison in a service-oriented architecture to provide maximum support for preservation planning endeavours. PLATO is a web based tool to help librarians, archivists, and curators weigh alternatives and decide which, if any, preservation actions to undertake for a specific set of records.” (Prom, 2010).

Plato’s decisions are based on the Requirements underlying the planning process. These

Requirements have the role of Constraints and are expressed as propositional statements in free-text. In order to help the requirements gathering process, they are organised in hierarchical

ObjectiveTrees with a high-level structure suggesting the break-down into Object Characteristics

(“Content”, “Context”, “Structure”, “Appearance” and “Behaviour”), Record Characteristics

describing the digital record, Process Characteristics describing the preservation process and Cost. But the tree structure has limited impact on the reasoning. The leaves of the tree branches are,

ideally, measurable and comparable criteria but may resort to subjective scales. It is possible, at this point to define the desirable Value of an extractable Characteristic rather than just a propositional statement.

Relationship to DePICT

Plato Constraints are fundamentally propositional and do not tie into a conceptual model. The emphasis, in Plato, is on the decision making process, rather than on the integrated conceptual modelling of the domain. Recently, Plato suggests that Characteristics are structured as

Object and Process Characteristics, but this distinction is only to organise the requirements gathering, rather than to structure the domain conceptually.

A propositional reasoning system is less expressive than the parameterised Object-Property-

Value model used in DEPICT. This means that concepts, elements and attributes of the Plato systems can be mapped onto the DEPICT model, but not necessarily the other way round.