ASTM E1432-91(1997)
(Practice)Standard Practice for Defining and Calculating Individual and Group Sensory Thresholds from Forced-Choice Data Sets of Intermediate Size
Standard Practice for Defining and Calculating Individual and Group Sensory Thresholds from Forced-Choice Data Sets of Intermediate Size
SCOPE
1.1 The definitions and procedures of this practice apply to the calculation of individual thresholds for any stimulus in any medium, from data sets of intermediate size, that is, consisting of more than 20 to 40 3-AFC presentations per individual. A group threshold may be calculated using 5 to 15 individual thresholds.
1.2 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: E 1432 – 91 (Reapproved 1997)
Standard Practice for
Defining and Calculating Individual and Group Sensory
Thresholds from Forced-Choice Data Sets of
Intermediate Size
This standard is issued under the fixed designation E 1432; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
The purpose of this practice is to determine individual sensory thresholds for odor, taste, and other
modalities and, when appropriate, calculate group thresholds. The practice takes as its starting point
any sensory threshold data set of more than 100 presentations, collected by a forced-choice procedure.
The usual procedure is the Three-Alternative Forced-Choice (3-AFC), as exemplified by Dynamic
Triangle Olfactometry. (Practice E 679 is suitable as a rapid method of determining an approximate
group threshold for limited-size data sets of 50 to 100 3-AFC presentations.)
Collection of the data is not a part of this practice. The data are assumed to be valid; for example,
it is assumed that the stimulus is defined properly, that each subject has been fully trained to recognize
the stimulus and did indeed perceive it when it was present above his or her momentary threshold, and
that the quality of dilution air did not vary. Assurance that the data are valid must be obtained (1)by
repeating the threshold determination several times and (2) by having the experimenter interview each
subject about the certainty of his or her perception.
It is recognized that precise threshold values for a given substance do not exist in the same sense
that values of vapor pressure exist. A panelist’s ability to detect a stimulus varies as a result of random
variations in factors such as alertness, attention, fatigue, events at the molecular level, etc., the effects
of which can usually be described in terms of a probability function. At low concentrations of an
odorant or tastant, the probability of detection by a given individual is typically 0.0 and at high
concentrations it is 1.0, and there is a range of concentrations in which the probability of detection is
between these limits. By definition, the threshold is the concentration for which the probability of
detection of the stimulus is 0.5 (that is, 50 % above chance, by a given individual, under the conditions
of the test).
Thresholds may be determined (1) for an individual (or for individuals one by one), and (2) for a
group (panel). While the determination of an individual threshold is a definable task, in which
precision of the result is mainly a question of letting the individual take enough tests at enough
concentrations, careful consideration for a group needs to be given to the additional question of what
the group is meant to represent.
The user must keep in mind the large degree of random error associated with estimating the
probability of detection from less than approximately 500 3-AFC presentations. (A presentation is a
set of samples, usually two, three, or five, judged as a unit by a single individual.) The reliance that
may be placed on the results can be increased greatly by enlarging the panel and by replicating the
tests.
1. Scope
1.1 The definitions and procedures of this practice apply to
the calculation of individual thresholds for any stimulus in any
This practice is under the jurisdiction of ASTM Committee E-18 on Sensory
Evaluation of Materials and Products and is the direct responsibility of Subcom-
medium, from data sets of intermediate size, that is, consisting
mittee E18.04 on Fundamentals of Sensory.
Current edition approved Aug. 15, 1991. Published October 1991.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E 1432 – 91 (1997)
of more than 20 to 40 3-AFC presentations per individual. A containing the added substance. (The 3-AFC procedure is
group threshold may be calculated using 5 to 15 individual different from the classical Triangle test, in which either one or
thresholds. two of the three samples may contain the added substance.)
1.2 This standard does not purport to address all of the
4.1.2 model—an abstract or concrete analogy, usually math-
safety concerns, if any, associated with its use. It is the
ematical, which represents in a useful way the functional
responsibility of the user of this standard to establish appro-
elements of a system or process. In short, the experimenter’s
priate safety and health practices and determine the applica-
theory of what is guiding the results observed.
bility of regulatory limitations prior to use.
4.1.3 statistical model—a model assuming that the principal
factor causing the results to deviate from the true value is a
2. Principles
random error process. This can usually be described in terms of
2.1 The 3-AFC procedure is one of the set of n-AFC
a probability function, for example, a bell-shaped curve,
procedures, any of which could be used, in principle, for the
symmetrical or skewed. Errors are binomially distributed in the
measurement of sensory thresholds, as could the duo-trio, the
3-AFC test procedure.
triangular, and the two-out-of-five procedures.
4.1.4 threshold, detection—the intensity of the stimulus that
2.2 For calculation of the threshold of one individual, this
has a probability of 0.5 of being detected under the conditions
practice requires data sets taken at five or more concentration
of the test. The probability of detection at any intensity is not
scale steps, typically six or seven steps, with each step differing
a fixed attribute of the observer, but rather a value which
from the previous step by a factor usually between 2 and 4,
assumes that sensitivity varies as a result of random fluctuation
typically 3.0. The practice presupposes that the range of
in factors such as alertness, attention, fatigue, and events at the
concentrations has been selected by pretesting, in order to
molecular level, the effects of which can be modeled by a
ensure that the individual’s threshold falls neither outside nor
probability function.
near the ends of the range, but well within it. At each
4.1.5 individual threshold—a threshold based on a series of
concentration step, the individual must be tested several times,
judgments by a single panelist.
typically five or more times.
4.1.6 group threshold—the average, median, geometric
2.3 Individual thresholds, as determined in 2.2, may be used
mean or other agreed measure (or an experimentally deter-
for calculation of a group (or panel) threshold. The size and
mined measure) of central tendency of the individual thresh-
composition of the panel (usually 5 to 15 members, preferably
olds of the members of a group (panel). The meaning and
more) is determined according to the purpose for which the
significance of the term depends on what the group is selected
threshold is required and the limitations of the testing situation
to represent (see 7.2.2).
(see 7.2).
4.1.7 scale step factor—for a scale of dilutions presented to
2.4 Pooling of the data sets from panel members to produce
a panel, the factor by which each step differs from adjacent
a single step calculation of the panel threshold is not permitted.
steps.
4.1.8 dilution factor—the following applies to flow olfacto-
3. Referenced Documents
metry: If F represents the flow of odorless gas which serves to
3.1 ASTM Standards:
dilute the flow of odorant, F , the dilution factor, Z, is given by:
E 122 Practice for Choice of Sample Size to Estimate a
F 1 F
1 2
Measure of Quality for a Lot in Process
Z 5 (1)
F
E 679 Practice for Determination of Odor and Taste Thresh-
olds by a Forced Choice Ascending Concentration Series
where Z is dimensionless. F and F may be expressed, both
1 2
Method of Limits
in units of mass, or (preferably) both in units of volume; the
3.2 French Standard:
report should state which. The term Z represents the dilution
NF X43-101, Air Quality; Method of Measuring the Odor
factor to threshold. The letter Z is used in honor of H.
Intensity of Gaseous Effluents; Determination of the
Zwaardemaker, a Dutch scientist and early investigator of
Dilution Factor to Perception Threshold
olfactometry. Alternate terminology in use is as follows:
dilution-to-threshold ratio (D/T or D-T); odor unit (OU); and
4. Terminology
effective dose (ED).
4.1 Definitions of Terms Specific to This Standard:
4.1.1 Three-Alternative Forced-Choice (3-AFC) test 5. Summary of Practice
procedure—a test presentation used in many threshold tests.
5.1 From a data set according to 2.2, calculate the threshold
For example, in odor testing by Dynamic Triangle Olfactom-
for one individual graphically or by linear regression according
etry, the panelist is presented with three gas streams, only one
to 5.2, or by using a model fitting computer program according
of which contains the diluted odorant, while the other two
to 5.3.
contain odorless carrier gas. The panelist must indicate the one
5.2 Obtain the threshold in 5.1 by first calculating the
proportion correct above chance for each concentration step.
This is accomplished by deducting, from the proportion of
Annual Book of ASTM Standards, Vol 14.02.
correct choices, the proportion that would have been selected
Annual Book of ASTM Standards, Vol 15.07.
by chance in the absence of the stimulus (see 8.1.2). Then, for
English translation available from AFNOR, Tour Europe, Cedex 7, 92080 Paris
La Défense, December 1986. each individual calculate that concentration which has a
E 1432 – 91 (1997)
probability of 0.5 of being detected under the conditions of the (2) That the sample drawn be truly random, that is, that
test. This is the individual threshold. every member of the population has a known chance of being
5.3 Alternatively obtain the threshold in 5.1 directly from selected, and
the proportion of correct choices by non-linear regression using
(3) That knowledge of the degree of variation occurring
a computer program, as described in 8.2.2.
within the population exists or can be acquired in the course of
5.4 Always report the individual thresholds of the panelists.
formulating the plan of sampling.
Depending on the purpose for which a threshold is required
7.2.2.1 In practice, the cost and availability of panelists
(see 7.2), and on the distribution found, a group threshold may
places serious limitations on the degree to which population
be calculated as the arithmetic or geometric mean, the median,
factors affecting thresholds, for example, age groups, gender,
or another measure of central tendency, or it may be concluded
ethnic origin, well versus ill, smoker versus nonsmoker, trained
that no group threshold can be calculated (see 7.4).
versus casual observers, etc., can be covered. The experimenter
is typically limited to panels of 5 to 15, with each receiving 20
6. Significance and Use
to 40 3-AFC presentations, for a total of 100 to 600 presenta-
tions. If the resulting thresholds are to have validity for the
6.1 Sensory thresholds are used to determine the potential of
population, the experimenter should include the following
substances at low concentrations to impart odor, taste, skinfeel,
steps:
etc. to some form of matter.
6.2 Thresholds are used, for example, in setting limits in air (1) Calculate and tabulate the thresholds for each indi-
pollution, in noise abatement, in water treatment, and in food vidual;
science and technology.
(2) Repeat the test for those individuals (outliers) falling
6.3 Thresholds are used to characterize and compare the
well beyond the range of the rest of the panel;
sensitivity of individuals or groups to given stimuli, for
(3) For any individuals whose threshold at first did not fall
example, in medicine, ethnic studies, and the study of animal
well within the range of samples presented to them, adjust the
species.
range and repeat the test; and
(4) If needed to obtain a desired level of precision, repeat
7. Panel Size and Composition Versus Purpose of Test
the test series with a second or third panel sampled from the
7.1 Panel Size and Composition—Panel variables should be same population of interest.
chosen as a function of the purpose for which the resulting
7.2.2.2 Thresholds vary with age, and one approach to the
threshold is needed. The important panel variables are as
question of a generalizable population value is to attempt to
follows:
estimate the threshold of healthy 20-year olds. According to
7.1.1 Number of tests per panelist,
Amoore (2), between the ages of 20 to 65, odor threshold
7.1.2 Number of panelists,
concentrations double for approximately each 22 years of age.
7.1.3 Selection of panelists to represent a given population,
7.2.3 The Distribution of Thresholds in the Population is
and
Required, for example, to determine what proportion of the
7.1.4 Degree of training.
population is affected by a given level of a pollutant, or,
7.2 Purpose of Test—It is useful to distinguish the following
conversely, to determine which concentrations of a pollutant
three categories:
will affect a given percent of a population. The requirements
7.2.1 Comparing an Individual’s Threshold With a Litera-
for testing are the same as in 7.2.2, except that it is even more
ture Value—The test may be conducted, for example, to
important to cover the range well, for example, to repeat the
diagnose anosmia or ageusia, or to study sensitivity to pain,
tests for those individuals whose thresholds fall in thinly
noise, or odor. This is the simplest category requiring a
populated parts of the panel range. Consideration should be
minimum of 20 to 40 3-AFC presentations to the individual in
given to increasing the number of presentations per concentra-
question (see 2.2). A number of training sessions may be
tion from 5-7 to 7-10 for such panel members. If the individual
required to establish the range of concentrations that will be
thresholds are plotted as in Fig. 1, any sector requiring study
used and to make certain that the individual is fully familiar
will be apparent from the graph.
with the stimulus to be detected as well as the mechanics of the
7.3 Trained Versus Casual Observers—Thresholds should
test.
normally be determined for observers trained by repeated
7.2.2 A Population Threshold is Required, for example, the
exposure to detect the stimulus in question whenever it is
odor threshold of a population exposed to a given pollutant, or
present; however, if the threshold sought is that of a casual
the flavor threshold of consumers of a beverage for a giv
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