ASTM E1432-04(2011)
(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<br> Intermediate Size
SIGNIFICANCE AND USE
Sensory thresholds are used to determine the potential of substances at low concentrations to impart odor, taste, skinfeel, etc. to some form of matter.
Thresholds are used, for example, in setting limits in air pollution, in noise abatement, in water treatment, and in food systems.
Thresholds are used to characterize and compare the sensitivity of individuals or groups to given stimuli, for example, in medicine, ethnic studies, and the study of animal species.
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 concerns, 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:E1432 −04 (Reapproved 2011)
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 E1432; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) 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
anysensorythresholddatasetofmorethan100presentations,collectedbyaforced-choiceprocedure.
TheusualprocedureistheThree-AlternativeForced-Choice(3-AFC)(seeISO13301),asexemplified
byDynamicTriangleOlfactometry.Asimilarpractice,PracticeE679,utilizeslimited-sizedatasetsof
50 to 100 3-AFC presentations, and is suitable as a rapid method to approximate group thresholds.
Collection of the data is not a part of this practice. The data are assumed to be valid; for example,
itisassumedthatthestimulusisdefinedproperly,thateachsubjecthasbeenfullytrainedtorecognize
thestimulusanddidindeedperceiveitwhenitwaspresentabovehisorhermomentarythreshold,and
that the quality of dilution medium did not vary.
It is recognized that precise threshold values for a given substance do not exist in the same sense
thatvaluesofvaporpressureexist.Apanelist’sabilitytodetectastimulusvariesasaresultofrandom
variations in factors such as alertness, attention, fatigue, events at the molecular level, health status,
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
probabilityofdetectionofthestimulusis0.5(thatis,50%abovechance,byagivenindividual,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, careful
consideration of the composition of the group is necessary to ensure the determined threshold
represents the group of interest.
Thereisalargedegreeofrandomerrorassociatedwithestimatingtheprobabilityofdetectionfrom
less than approximately 500 3-AFC presentations. The reliability of the results can be increased
greatly by enlarging the panel and by replicating the tests.
1. Scope medium, from data sets of intermediate size, that is, consisting
of more than 20 to 40 3-AFC presentations per individual. A
1.1 The definitions and procedures of this practice apply to
group threshold may be calculated using 5 to 15 individual
the calculation of individual thresholds for any stimulus in any
thresholds.
1.2 This standard does not purport to address all of the
This practice is under the jurisdiction of ASTM Committee E18 on Sensory
Evaluation and is the direct responsibility of Subcommittee E18.04 on Fundamen-
safety concerns, if any, associated with its use. It is the
tals of Sensory.
responsibility of the user of this standard to establish appro-
Current edition approved Aug. 1, 2011. Published August 2011. Originally
priate safety and health practices and determine the applica-
approved in 1991. Last previous edition approved in 2004 as E1432–04. DOI:
10.1520/E1432-04R11. bility of regulatory limitations prior to use.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1432−04 (2011)
2. Principles one containing the added substance. (The 3-AFC procedure is
differentfromtheclassicalTriangletest,inwhicheitheroneor
2.1 The 3-AFC procedure is one of the set of n-AFC
two of the three samples may contain the added substance.)
procedures, any of which could be used, in principle, for the
4.1.2 model—an abstract or concrete analogy, usually
measurement of sensory thresholds, as could the duo-trio, the
mathematical, which represents in a useful way the functional
triangular, and the two-out-of-five procedures.
elements of a system or process. In short, the experimenter’s
2.2 For calculation of the threshold of one individual, this
theory of what is guiding the results observed.
practice requires data sets taken at five or more concentration
4.1.3 statistical model—amodelassumingthattheprincipal
scalesteps,typicallysixorsevensteps,witheachstepdiffering
factor causing the results to deviate from the true value is a
from the previous step by a factor usually between 2 and 4,
randomerrorprocess.Thiscanusuallybedescribedintermsof
typically 3.0. The practice presupposes that the range of
a probability function, for example, a bell-shaped curve,
concentrations has been selected by pretesting, in order to
symmetricalorskewed.Errorsarebinomiallydistributedinthe
ensure that the individual’s threshold falls neither outside nor
3-AFC test procedure.
near the ends of the range, but well within it. At each
concentration step, the individual must be tested several times,
4.1.4 threshold, detection—the intensity of the stimulus that
typically five or more times.
has a probability of 0.5 of being detected under the conditions
of the test. The probability of detection at any intensity is not
2.3 Individualthresholds,asdeterminedin2.2,maybeused
a fixed attribute of the observer, but rather a value which
for calculation of a group (or panel) threshold. The size and
assumesthatsensitivityvariesasaresultofrandomfluctuation
composition of the panel (usually 5 to 15 members, preferably
infactorssuchasalertness,attention,fatigue,andeventsatthe
more) is determined according to the purpose for which the
molecular level, the effects of which can be modeled by a
threshold is required and the limitations of the testing situation
probability function.
(see 7.2).
4.1.5 individual threshold—a threshold based on a series of
2.4 Pooling of the data sets from panel members to produce
judgments by a single panelist.
asinglestepcalculationofthepanelthresholdisnotpermitted.
3. Referenced Documents
3.1 ASTM Standards:
E122PracticeforCalculatingSampleSizetoEstimate,With
Specified Precision, the Average for a Characteristic of a
Lot or Process
E679Practice for Determination of Odor and Taste Thresh-
olds By a Forced-ChoiceAscending Concentration Series
Method of Limits
3.2 CEN Standard:
EN13725AirQuality—DeterminationofOdourConcentra-
tion Using Dynamic Dilution Olfactometry
3.3 ISO Standard:
ISO 13301 Sensory Analysis—Methodology—General
guidance for Measuring Odour, Flavour, and Taste Detec-
tion Thresholds by a Three Alternative Forced Choice
(3-AFC) Procedure
4. Terminology
4.1 Definitions of Terms Specific to This Standard:
4.1.1 Three-Alternative Forced-Choice (3-AFC) test
procedure—a test presentation used in many threshold tests.
For example, in odor testing by Dynamic Triangle
Olfactometry, the panelist is presented with three gas streams,
onlyoneofwhichcontainsthedilutedodorant,whiletheother
twocontainodorlesscarriergas.Thepanelistmustindicatethe
NOTE 1—This probability graph shows 20 panelists sorted by rank as
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
described in 9.3.2. Data are adapted from French Standard X43-101.
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Group threshold=T=50% point=log(Z )=2.32. Group standard de-
Standards volume information, refer to the standard’s Document Summary page on
viation from% and 84% points=σ=(3.07−1.57)⁄2=0.75 in log(Z)
the ASTM website.
units. The 99% point is off the graph but can be calculated as
Available from British Standards Institution (BSI), 389 Chiswick High Rd.,
2.32+(0.75×2.327)=4.07,where2.327isthe%pointontheabscissaof
London W4 4AL, U.K., http://www.bsigroup.com.
the normal curve of error.
Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
4th Floor, New York, NY 10036, http://www.ansi.org. FIG. 1Group Threshold by Rank-Probability Graph
E1432−04 (2011)
4.1.8 dilution factor—the following applies to flow olfacto-
metry:IfF representstheflowofodorlessgaswhichservesto
dilutetheflowofodorant,F ,thedilutionfactor,Z,isgivenby:
F 1F
1 2
Z 5 (1)
F
where Z is dimensionless. F and F may be expressed,
1 2
both in units of mass, or (preferably) both in units of vol-
ume; the report should state which. The term Z represents
the dilution factor to threshold. Alternate terminology in use
is as follows: dilution-to-threshold ratio (D/T or D-T); odor
unit (OU); and effective dose (ED).
5. Summary of Practice
5.1 From a data set according to 2.2, calculate the threshold
FIG. 2 Symmetrical, Bell-Shaped Distribution
foroneindividualgraphicallyorbylinearregressionaccording
to5.2,orbyusingamodelfittingcomputerprogramaccording
to 5.3.
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
correct choices, the proportion that would have been selected
by chance in the absence of the stimulus (see 8.1.2). Then, for
each individual calculate that concentration which has a
probability of 0.5 of being detected under the conditions of the
test. This is the individual threshold.
5.3 Alternatively obtain the threshold in 5.1 directly from
theproportionofcorrectchoicesbynon-linearregressionusing
a computer program, as described in 8.2.2.
5.4 Always report the individual thresholds of the panelists.
Depending on the purpose for which a threshold is required
FIG. 3 Skewed Distribution
(see 7.2), and on the distribution found, a group threshold may
be calculated as the arithmetic or geometric mean, the median,
oranothermeasureofcentraltendency,oritmaybeconcluded
that no group threshold can be calculated (see 7.4).
6. Significance and Use
6.1 Sensorythresholdsareusedtodeterminethepotentialof
substancesatlowconcentrationstoimpartodor,taste,skinfeel,
etc. to some form of matter.
6.2 Thresholds are used, for example, in setting limits in air
pollution, in noise abatement, in water treatment, and in food
systems.
6.3 Thresholds are used to characterize and compare the
sensitivity of individuals or groups to given stimuli, for
example, in medicine, ethnic studies, and the study of animal
species.
FIG. 4 Bi-Modal Distribution
7. Panel Size and Composition Versus Purpose of Test
4.1.6 group threshold—the average, median, geometric
mean or other agreed measure (or an experimentally deter-
7.1 Panel Size and Composition—Panel variables should be
mined measure) of central tendency of the individual thresh-
chosen as a function of the purpose for which the resulting
olds of the members of a group (panel). The meaning and
threshold is needed. The important panel variables are as
significance of the term depends on what the group is selected
follows:
to represent (see 7.2.2).
7.1.1 Number of tests per panelist,
4.1.7 scale step factor—for a scale of dilutions presented to 7.1.2 Number of panelists,
a panel, the factor by which each step differs from adjacent 7.1.3 Selection of panelists to represent a given population,
steps. and
E1432−04 (2011)
NOTE 1—The results (using Probits and linear regression) are as follows:
Panelist No. 1 2 3 4 5 6
Threshold, ppb 381 166 226 97 47 12
Group standard deviation (six panelists), σ = 0.539 in log (ppb) units.
FIG. 5Graphic Estimation of Approximate Thresholds for the Six Panelists in 7.3
7.1.4 Degree of training. the flavor threshold of consumers of a beverage for a given
contaminant. In this case, recourse must be had to the rules of
7.2 PurposeofTest—Itisusefultodistinguishthefollowing
sampling from a population (see Ref (1) and Practice E122),
three categories:
which require the following:
7.2.1 Comparing an Individual’s Threshold With a Litera-
(1) That the population be accurately defined and
ture Value—The test may be conducted, for example, to
delimited,
diagnose anosmia or ageusia, or to study sensitivity to pain,
(2)That the sample drawn be truly random, that is, that
noise, or odor. This is the simplest category requiring a
every member of the population has a known chance of being
minimum of 20 to 40 3-AFC presentations to the individual in
selected, and
question (see 2.2). A number of training sessions may be
(3)That knowledge of the degree of variation occurring
required to establish the range of concentrations that will be
withinthepopulationexistsorcanbeacquiredinthecourseof
used and to make certain that the individual is fully familiar
formulating the plan of sampling.
withthestimulustobedetectedaswellasthemechanicsofthe
test.
7.2.2 A Population Threshold is Required, for example, the
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
odor threshold of a population exposed to a given pollutant, or this standard.
E1432−04 (2011)
NOTE 1—The PROC NLIN fits nonlinear regression models by least squares. Following the regression expression, the operator selects one of four
iterative methods (here, DUD) and must specify an approximate value for the parameters B (the slope, here=−4) and T (the threshold, here=2). The
NLIN procedure first prints out the starting values for B and T, then proceeds stepwise (here, ten steps) until the residual sum of squares no longer
decreases (“convergence criterion met”). The threshold (here, T=log(ppb)=1.954) is found as the last value in the T column. The results for the six
panelists are as follows:
Panelist 1 2 3 4 5 6
Method DUD DUD DUD DUD DUD MARQUARDT
Log (ppb) 2.518 2.249 2.368 1.954 1.806 0.892
Threshold, ppb 330 178 249 90 64 7.8
Group standard deviation (six panelists),σ = 0.59 in log(ppb) units.
FIG. 6Output from SAS NLIN Program (6) with Details for Panelist No. 4
E1432−04 (2011)
7.2.2.1 In practice, the cost and availability of panelists point of a log-probability graph (see Fig. 1) is the appropriate
places serious limitations on the degree to which population measure. Conversion of the concentration scale into double
factors affecting thresholds, for example, age groups, gender, logarithms (log of log) is occasionally needed to normalize a
ethnicorigin,wellversusill,smokerversusnonsmoker,trained distribu
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