ISO 21043-4:2025

International
Standard
ISO 21043-4
First edition
Forensic sciences —
2025-06
Part 4:
Interpretation
Criminalistique —
Partie 4: Interprétation
Reference number
© ISO 2025
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 General requirements . 1
5 Questions and propositions . 2
5.1 Questions .2
5.1.1 General .2
5.1.2 Types of questions.2
5.1.3 Classification, including identification .2
5.1.4 Quantification .2
5.1.5 Question of source .3
5.1.6 Reconstruction . . .3
5.2 Addressing the question with either investigative or evaluative interpretation .4
5.2.1 General .4
5.2.2 Investigative interpretation .4
5.2.3 Evaluative interpretation .4
6 Types of opinions . 4
6.1 General .4
6.2 Likelihood ratios .5
6.3 Categorical opinions .5
6.4 Explanations .5
6.5 Estimations .5
7 Assigning probabilities . 6
7.1 General .6
7.2 Statistical-model-based assignment of probabilities .6
7.3 Human-based assignment of probabilities .6
8 Expression of the opinion . 6
8.1 General .6
8.2 Quantitative expression of opinion .6
8.3 Qualitative expression of opinion .7
8.4 Categorical opinions .7
Annex A (informative) Likelihood ratio . 8
Annex B (informative) Examples of opinion scales . 9
Annex C (informative) Examples of interpretation .11
Bibliography . 14

iii
Foreword
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in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
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This document was prepared by Technical Committee ISO/TC 272, Forensic sciences, in collaboration with
the European Committee for Standardization (CEN) Technical Committee CEN/TC 419, Forensic science
processes, in accordance with the Agreement on technical cooperation between ISO and CEN (Vienna
Agreement).
A list of all parts in the ISO 21043 series can be found on the ISO website.
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
This document is part of a series of includes the different components of the forensic process from scene to
courtroom. Other parts of the ISO 21043 series are:
— ISO 21043-1;
— ISO 21043-2;
— ISO 21043-3;
— ISO 21043-5.
Annexes A, B and C are given for information only.

v
International Standard ISO 21043-4:2025(en)
Forensic sciences —
Part 4:
Interpretation
1 Scope
This document specifies requirements and provides recommendations for the interpretation of observations
to reach opinions that answer questions that are relevant for decision making in investigations or legal
proceedings. This document states requirements that are applicable to all forensic disciplines. This
document states requirements that apply when the opinion is based directly on human judgement and
when the opinion is based on interpretation through a statistical model. This document is applicable to
interpretation that occurs at a scene, within a facility, or within a judicial setting.
This document establishes requirements designed to safeguard the process for the interpretation of
observations including the use of either statistical models or human judgement, to address alternative
propositions based on the questions asked by the customer.
Interpretation is not necessary and the requirements of this document do not apply if the observations
resulting from the analysis directly answer the relevant question.
EXAMPLE In analytical chemistry, substances are often identified or classified. Provided that the applied
analytical methods are not limited in selectivity or sensitivity for the given question, the observations can lead
to a direct statement of the name of the substance (identification) or a type of material (classification). This is not
considered interpretation for the purposes of this document.
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 21043-1, Forensic sciences — Part 1: Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 21043-1 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/
4 General requirements
Forensic service providers shall have documented methods for interpretation.
The interpretation methods shall be suitable for the intended use and should be supported by relevant
validation studies.
Any deviation from a documented interpretation method shall be technically justified and recorded.

Interpretation shall:
a) be relevant to the question put forward by the customer;
b) be supported by task-relevant data, observations and information that are within the examiner’s area of
expertise;
c) take into account
...