ISO 13909-8:2025

International
Standard
ISO 13909-8
Third edition
Coal and coke — Mechanical
2025-08
sampling —
Part 8:
Methods of testing for bias
Charbon et coke — Échantillonnage mécanique —
Partie 8: Méthodes de détection du biais
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 Principles . 1
5 Pretest inspection . 2
6 Reference methods . 2
7 Test design . 3
7.1 Choice of test parameters .3
7.2 Number of paired samples .3
7.3 Selection of sample pairs .3
7.3.1 Composition of sample pairs .3
7.3.2 Paired-increment samples .4
7.3.3 Paired-batch samples .4
7.4 Choice of fuel for test .4
7.5 Coke .4
8 Conduct of the test . 4
8.1 General .4
8.2 Collection and preparation of test samples .5
9 Outline of test procedure . 5
9.1 General .5
9.2 Special precautions for moisture-test samples .5
9.3 Documentation .6
10 Statistical analysis and interpretation . 6
10.1 Outline of statistical procedure .6
10.2 Calculations .6
10.2.1 Statistical procedure for identifying outliers .6
10.2.2 Disposition of outliers .7
10.2.3 Calculation of confidence intervals and determining bias .8
10.2.4 Review of the 95 % confidence region for the bias .11
11 Test report .11
Annex A (informative) Example calculations.13
Bibliography .26

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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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 document 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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Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 27, Coal and coke, Subcommittee SC 4, Sampling.
This third edition cancels and replaces the second edition (ISO 13909-8:2016), which has been technically
revised.
The main changes are as follows:
— the main title has been modified and aligned with the ISO 13909 series;
— requirements in reference methods have been clarified.
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
It is not possible to lay down a standard method for field work by which a sampling procedure can be tested
for bias, because details of the procedure will inevitably be affected by local conditions. However, certain
principles can be specified which ought to be adhered to whenever possible and these are discussed in this
document.
Testing for bias can be a tedious and expensive process. All bias tests therefore include a thorough pretest
inspection, with appropriate action taken regarding any system deficiencies likely to cause bias.
In the text, the term “fuel” is used where both coal and coke would be applicable in the context and either
“coal” or “coke” where only one is applicable.

v
International Standard ISO 13909-8:2025(en)
Coal and coke — Mechanical sampling —
Part 8:
Methods of testing for bias
1 Scope
This document sets out principles and procedures for testing the bias of test samples of coals or cokes, taken
according to the ISO 13909 series.
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 13909-1, Coal and coke — Mechanical sampling — Part 1: General introduction
ISO 13909-4, Coal and coke — Mechanical sampling — Part 4: Preparation of test samples of coal
ISO 13909-6, Coal and coke — Mechanical sampling — Part 6: Preparation of test samples of coke
ISO 13909-7, Coal and coke — Mechanical sampling — Part 7: Methods for determining the precision of sampling,
sample preparation and testing
ISO 21398, Hard coal and coke — Guidance to the inspection of mechanical sampling systems
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 13909-1 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 Principles
The testing of a sampling system for bias is based on taking a series of pairs of samples of essentially the same
fuel; one member of each pair being sampled by the system or component under test, the other member being
obtained by a reference method. For each pair, the difference between the analytical results is determined.
The series of differences between the analytical results thus obtained are subjected to statistical analysis.
The sensitivity of the statistical test is dependent on the number of pairs compared, the variability of the
differences between members of the pairs, and the number of parameters used for the test.
The statistical analysis to which results will be subjected assumes the following three conditions:
— a multivariate normal distribution of the variables;
— independence of the errors of measurement for the individual parameters;

— homogeneity of the data.
5 Pretest inspection
The primary sources of information regarding compliance with the sampling standard are the equipment
specifications and drawings.
A thorough examination of the sampling system and a review of its component specification shall be made,
in accordance with ISO 21398.
The party performing the test shall, however, verify performance by field measurements and observations.
The operation of the sampling system shall be observed both dynamically with fuel flowing and statically
with no fuel flowing. Some components will need to be actuated in static mode.
Pretest inspections of all operations and equipment, both static and under load, shall be carried out
by persons experienced in the sampling of segregated, heterogeneous, and lumpy bulk materials. It is
recommended that operation under normal conditions be observed for an entire lot.
Do not execute a test for bias until all conditions known to cause bias are corrected unless it is necessary to
establish the performance of a system or component as it stands. In the latter case, the pretest inspection
provides essential documentation of what the conditions were at the time of the test.
6 Reference methods
To test overall system bias, the use of a reference method which is known to be intrinsically unbiased is
required. The preferred method is the stopped-belt method, i.e. the collection of increments from a complete
cross-section of the fuel on the conveyor belt by stopping the belt at intervals. When properly collected from
the primary fuel stream, the stopped-belt increment can be considered as a reference increment.
NOTE If it is not possible to collect stopped-belt increments, other reference methods can be used but, in these
cases, an apparent absence of a lack of bias relative to the reference method can be inconclusive and the use of such
methods can compromise the validity and authority of the findings.
With collection of stopped-belt increments, some disruption of normal operations can occur, and therefore,
the plan of execution may need to be coordinated with the normal operations and organized to minimize
such disruption. It must be recognized that the conveyor system involved may be used for only a few hours
per day for normal operations and cannot be operated solely for bias tests unless the fuel can be diverted
to another discharge point. This can extend the time necessary for completion of the field work and require
special arrangements for supplying fuel to the system for testing.
Stopped-belt increments shall be taken with a sampling frame (see Figure 1), or equivalent, from a complete
cross-section of the solid mineral fuel on the belt at a fixed position, for a length along the belt which is at
least three times the nominal top size of the fuel.
Figure 1 — Sampling frame
The sampling frame (or equivalent) shall be placed on the stationary belt at the predetermined position
so that the separator plates at each end are in contact with the belt across its full width. All particles lying
inside the sampling frame end plates shall be swept into the sampling container.
Particles obstructing the insertion of the end plate on the left-hand side shall be pushed into the increment,
while those obstructing the insertion of the end plate on the right-hand side shall be pushed out of the
increment or vice-versa. Whichever practice is used initially, this practice shall be implemented throughout
the test.
Care shall be taken to minimize the risk of bias being introduced in the course of preparation of the
increments and samples and all sample division equipment and procedures shall be checked for bias with
respect to relevant test parameters.
It is recommended that the mass of all increments/samples be determined immediately after collection. Pay
close attention to minimizing unintended mass losses.
Report all observed mass losses.
7 Test design
7.1 Choice of test parameters
The standard test for general purposes shall include moisture and ash (ash on a dry basis). Use of these
two parameters will generally suffice. Other variables can be included if desired. However, given a fixed
number of sample pairs, the test often becomes less sensitive to detection of a bias as additional variables
are included.
Bias in ash on a dry basis is most commonly caused by errors in size distribution. Bias in moisture content
may be caused by a wide variety of factors, including, but not limited to, errors in size distribution, moisture
losses associated with crushers, excessive ventilation within the sampling system, less than the closest
possible coupling between system components, excessive retention time in the system, or any combination
of these.
Direct tests on particle size distribution are often necessary for coke. It is recommended that tests for size
distribution be conducted as a separate test and that size distribution parameters not be included in the
standard test for analytical parameters.
7.2 Number of paired samples
The minimum number of paired samples for the test shall be 30. However, if necessary, more sets may be
collected if the variance of the quality of the fuel is suspected to influence the detection of bias. On the other
hand, if use of a smaller number of paired samples results in a detection of bias, no additional samples need
be collected. After completing the statistical calculations, the resulting confidence region can be evaluated.
If no bias is detected, yet the confidence region covers levels of bias that are of commercial concern, more
paired samples can be collected for coal originating from this same source in order to reduce the size of the
confidence region.
7.3 Selection of sample pairs
7.3.1 Composition of sample pairs
The members of each pair of samples can each be comprised of portions of one or more increments.
Individual increments can be paired or samples of compounded increments can be paired. The test shall be
structured so that the expected mean of the differences of the result would be zero if no systematic error is
present in the system under test.

7.3.2 Paired-increment samples
Paired-increment experimental design is the comparing of individual primary increments after being
processed by the system, with the reference samples collected from the stopped belt.
NOTE 1 A final system sample consisting only of fuel collected from a single primary increment can be of
insufficient mass to provide an analytical result following the requirements specified in ISO 13909-4 and ISO 13909-6
for preparation and analysis by the methods used routinely during regular operations.
NOTE 2 In causing the mechanical system to separately collect and process individual primary increments for a
test for bias, the system sample can lose more moisture than while the system is operating as designed.
7.3.3 Paired-batch samples
It is often not practical to obtain single increment samples from the system. Increments taken by the system
can be compounded as samples and paired with samples compounded from increments taken over the same
period using the reference method. It is not necessary that the two sample members, reference and system,
have the same number of increments or be of similar mass. Single stopped-belt reference increments are
often used as the reference sample and paired with compounded system sample increments. In collecting
paired-batch samples, the timing at which stopped-belt increments are collected from each moving batch
shall be determined using a random systematic sampling scheme.
7.4 Choice of fuel for test
If more than one coal is to be sampled by the system, the coal chosen for the test shall be one that is expected
to show up any bias in the sampling system. For example, bias for ash on primary samplers and sample
dividers is commonly caused by the exclusion of larger sized particles. If a coal is chosen where the ash of
such particles is similar to that of the coal as a whole, yet the particles are too large for collection, then no
bias will be detected even though those particles are being excluded. If subsequently, the sampler is used to
sample a coal where the large particles have an ash which differs from the mean, the sampling system can be
biased by not accurately representing those particles.
It is recommended that the complete bias test be carried out on coal from a single source.
7.5 Coke
The same requirements as those specified in 7.4 for coal with respect to its ash shall apply when choosing a
coke for test with respect to its moisture content.
8 Conduct of the test
8.1 General
Test the whole system by comparing stopped-belt reference samples taken from the primary flow with
the product at the final stage of the preparation system. A bias test for the whole system is carried out by
comparing the reference samples taken from the main flow with the samples collected at the final stage of
the on-line system. The paired batch experimental design (see 7.3.3) is the preferred practice because it
minimizes the disruption of normal operations.
Bias may be hardware induced, system logic induced, or a combination of both. It follows that routine
operating conditions are best simulated by operating sampling systems under the control of system logic at
routine operating condition settings. When using the paired increment methodology, do not stop the rest of
the downstream system of the primary sampler via the system interlocks. This can be facilitated by use of a
bias-test mode of system operation. This is not an issue in paired batch tests.
Conveyor belt systems for handling fuel are often not designed for repeated starting and stopping under
load. The paired increment experimental design does not necessarily preclude collection of stopped-belt
increments, provided arrangements are made to stop the feed to the belt from which the increment is to be
collected before the belt is stopped, so that the belt will be only partially loaded on restart. Increments can

then be collected off the belt from the points in the stream where conditions that prevailed before the feed
was stopped still exist.
8.2 Collection and preparation of test samples
Special care is required in planning the duties of each member of the sampling team and exactly how such
duties will be performed. At this stage of planning, detailed operating protocols shall be established to
ensure uniform and consistent collection, mass determination and processing of samples. This includes the
preparation of facilities and assembly of all equipment necessary for collection of samples, processing and
packaging them, and transporting them from collection and mass determination sites to processing facilities
and to the laboratory for analysis.
Special safety precautions are necessary for personnel working around the fuel handling and sampling
machinery. Attention is drawn to the need to comply with all relevant safety regulations, especially the
blocking of drive power to the conveyor from which the reference samples are collected.
The efficiency of the matched-pairs experimental design depends on the closeness with which reference
and system samples are physically paired (correlated) to minimize the impact of variations in product
quality within pairs. The extent to which within-pair variances are smaller than between-pair variances is
an indication that the objective of the matched-pairs experimental design is being fulfilled. Prepare member
pairs of samples together (even though through different steps) and analyse them in the same batch to avoid
the introduction of systematic error resulting from variations in treatment during preparation and analysis.
Be certain to preserve the identity of all samples.
Check the nominal top size of the fuel and the product of any sample crusher that is an integral part of the
system or subsystem under test. Include this information in the test report. Samples for this purpose shall
be separate from the matched pairs taken for the bias test.
In conducting bias tests where moisture content is a test parameter, care shall be take
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