ISO/PAS 25955:2026

Publicly
Available
Specification
ISO/PAS 25955
First edition
Information and documentation —
2026-03
Technical interoperability — Data
Documentation Initiative (DDI)
Information et documentation — Interopérabilité technique —
Initiative de documentation des données (DDI)
Reference number
© ISO 2026
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Metadata . 2
5 Variables . 2
6 Variable cascade . 2
7 Unit cascade . 4
8 Value domains . 4
8.1 General .4
8.2 Value domain structures .4
8.2.1 General .4
8.2.2 Enumerated value domains .4
8.2.3 Described value domain — Range .5
8.2.4 Described value domain — Rule .5
8.3 Value domain usages .5
9 Data lifecycle . 6
10 Model independence . 7
11 Semantic interoperability . 7
Bibliography . 8

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
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with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
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patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
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This document was prepared by Technical Committee ISO/TC 46, Information and documentation,
Subcommittee SC 4, Technical interoperability.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
The Data Documentation Initiative (DDI) is an ongoing program within the social, behavioural and economic
(SBE) data community for documenting SBE data. The program is managed by a consortium, the DDI
[6]
Alliance , that comprises university data libraries, university and national data archives, research centres,
national statistical offices, consultancies, and software development organizations. The suite of products
under DDI include several standards and other products designed to describe data and the processes used
to produce them.
DDI first began in 1995 to build a framework for describing a codebook, the document describing the
variables, questions, code lists, classifications, methodologies, and usage guidance used to collect, organize,
and delimit the data in research studies. This document was built using XML (eXtensible Markup Language)
[8]
and first released in 2001 as DDI-Codebook (DDI-C) . It has been updated periodically since and the current
version is numbered 2.5.
In 2003, the DDI community realized an expanded version of DDI-C that addressed the descriptive needs of
data producers, such as national statistical offices, was needed. These requirements included descriptions
of the methodologies used in production, which include questionnaires, sampling, and weighting. These are
used in probability based statistical sample surveys, the standard design for surveys conducted by national
statistical offices. . Further, the ability to reuse descriptions of variables, questions, and other artefacts
was considered essential for efficiently documenting ongoing statistical surveys and for supporting
comparability, interoperability, and integration over time. This effort, also developed using XML, was called
[11]
DDI-Lifecycle (DDI-L). Its current version is numbered 3.3.
Reacting to a dramatic decline in response rates to their surveys, survey organizations began looking into
enhancing their data by integrating them with data from outside sources. These data come in a variety
of formats, with inconsistent quality and coverage, and often aren’t based on a probability sample. A
framework of descriptions, intended to be independent of SBE and to support data integration, was designed
to address this problem. The result was the DDI Alliance developed the Cross-Domain Integration (DDI-CDI)
[9]
standard . It was released in early 2025.
Along with the standards, the DDI Alliance advocated the development of several supporting products,
which are:
[13]
— XKOS – eXtended Knowledge Organization System is an extension of the W3C SKOS (Simple Knowledge
Organization System) used to render concept systems in RDF (Resource Description Framework). XKOS
extends SKOS in two major ways, it
— supports levels in hierarchies, which are used to organize statistical classifications
— includes the semantics for some temporal and sequential relations;
[12]
— SDTL – Structured Data Transformation Language is a mid-level language for documenting the
processing steps in a data production environment;
[9]
— Controlled Vocabularies – A series of category sets and code lists for use as common ways to populate
some descriptors in the DDI standards. An example is the names for the kinds of sampling schemes
available.
Beginning around 1970, several researchers independently coined the term metadata. This loosely meant
“data about data”, and it referred to descriptions of data. Since then, the concept of metadata has expanded,
and the term is now applied to descriptions of any object or resource, not just to data. This expanded notion
of metadata is how the term is understood. Each of the DDI standards describes more than data.
Given this understanding of metadata, all the DDI standards are metadata standards. They address the
organization and formats of the metadata needed for describing data and related artefacts. This mostly
applies to SBE data, but the DDI-CDI standard addresses data independent of the source.
This document describes several common aspects of all the DDI standards and other products. The DDI
standards all use the notion of a variable, and these roughly correspond to a column of data organized in the
rectangular format. The focus is on the common notion of a variable and the organization of its description.

v
Each DDI standard and the other products are used to describe parts of the data lifecycle. The phases of this
lifecycle are defined, and which phases are addressed by the DDI standards and products are identified.
Of the DDI standards managed using a UML (Unified Modelling Language) model now, those models are
independent of each syntax representation (for example, XML, JSON, RDF, etc.) and are called Platform
Independent Models (PIM). Each syntax representation uses its own model, and each is an approximation
(as close as possible) to the PIM. They are called Platform Specific Models (PSM).
Using the terminology and ideas in ISO/IEC Guide 2, each PSM conforms to its PIM.

vi
Publicly Available Specification ISO/PAS 25955:2026(en)
Information and documentation — Technical interoperability
— Data Documentation Initiative (DDI)
1 Scope
This document describes the shared and interoperable features of the standards and other products under
the Data Documentation Initiative (DDI).
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 1087:2019, Terminology work and terminology science — Vocabulary
ISO/IEC Guide 2:2004, Standardization and related activities — General vocabulary
DDI Alliance (2025). DDI Glossary v1.0 Available from: https:// ddialliance .org/ glossary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 1087, ISO/IEC Guide 2, the DDI
Glossary and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
concept
unit of thought differentiated by characteristics
3.2
platform independent model
PIM
document describing the abstract model of the standardized data exchange process in a platform-
independent way
Note 1 to entry: The specific technological platform may be but not limited to a programming language or a database.
[SOURCE: ISO 10303-2:2004, 3.16.12]
3.3
platform specific model
PSM
model of a software system or business system linked to a specific technological platform
Note 1 to entry: The specific technological platform may be but not limited to a programming language, operating
system, document file format, or database.
[SOURCE: ISO 10303-2:2004, 3.16.13]

4 Metadata
For the DDI standards and related products, metadata are data describing some object(s), and all the
standards under DDI are frameworks for organizing metadata. Some metadata are machine-actionable, and
others are just readable as text. Machine-actionable metadata are those in a format that can be read and
processed using predefined code and logic.
Machine-actionable metadata are desirable, but because of the broad scope of the DDI standards, not all the
metadata they prescribe are machine-actionable.
5 Variables
A variable is a mapping between a set of units called a sample and a set of permissible (or allowed) values
called a value domain. A unit (see DDI Glossary) is an object (as defined in ISO 1087), and each unit in t
...