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Significance and Use
Under the assumptions of the model, the Thurstonian model approach to measuring the perceived difference between two samples (whether overall or for a specific attribute) is independent of the sensory method used to collect the data. Converting results obtained from different test methods to d' values permits the assessment of relative differences among samples without requiring that the samples be compared to each other directly or that the same test methods be used for all pairs of samples.
Thurstonian scaling has been applied to:
Creating a historical database to track differences between production and reference samples over periods in which different test methods were used to measure the difference,
Comparing the relative sensitivities of different user groups and consumer segments,
Comparing trained panels that use different measuring techniques,
Comparing the relative sensitivities of consumers versus trained panels,
Comparing different methods of consumer testing (for example, CLT versus HUT, preference versus hedonic scales, etc.), and
Comparing different discrimination test methods.
1.1 This practice describes procedures to estimate Thurstonian discriminal distances (that is, d' values) from data obtained on two samples. Procedures are presented for four forced-choice methods (that is, the triangle, the duo-trio, the 3-alternative-forced-choice (or 3-AFC) and the 2-AFC (also called the directional difference test)), the A/Not-A method, the Same-Different method and for data obtained from ordered category scales. Procedures for estimating the variance of d' are also presented. Thus, confidence intervals and statistical tests can be calculated for d'.
1.2 The procedures in this document pertain only to the unidimensional, equal-variance model. Other, more complicated Thurstonian models, involving multiple dimensions and unequal variances exist but are not addressed in this standard. The procedure for forced-choice methods is limited to dichotomous responses. The procedure for the A/Not-A method assumes equal sample sizes for the two samples. The procedure for the Same-Different method assumes equal sample sizes for the matched and unmatched pairs of samples. For all methods, only unreplicated tests are considered. (Tests in which each assessor performs multiple (that is, replicated) evaluations require different analyses.)
1.3 Thurstonian scaling is a method for measuring the perceptual difference between two samples based on a probabilistic model for categorical choice decision making. The magnitude of the perceived difference, δ, can be estimated from the assessors' categorical choices using the methods described in this practice (See Appendix X3 for a more detailed description of Thurstonian scaling).
1.4 In theory, the Thurstonian δ does not depend on the method used to measure the difference between two samples. As such, δ provides a common scale of measure for comparing samples measured under a variety of test conditions. For example, Thurstonian scaling can be used to compare products measured under different test conditions, to compare panels (trained, consumer or both) that have evaluated the same samples (using the same or different test methods) and to compare test methods on their ability to discriminate samples that exhibit a fixed sensory difference.
1.5 This standard may involve hazardous materials, operations and equipment. 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.
2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.
E253 Terminology Relating to Sensory Evaluation of Materials and Products
E456 Terminology Relating to Quality and Statistics
E460 Practice for Determining Effect of Packaging on Food and Beverage Products During Storage
E679 Practice for Determination of Odor and Taste Thresholds By a Forced-Choice Ascending Concentration Series Method of Limits
E1432 Practice for Defining and Calculating Individual and Group Sensory Thresholds from Forced-Choice Data Sets of Intermediate Size
E1593 Guide for Assessing the Efficacy of Air Care Products in Reducing Sensorly Perceived Indoor Air Malodor Intensity
E1627 Practice for Sensory Evaluation of Edible Oils and Fats
E1697 Test Method for Unipolar Magnitude Estimation of Sensory Attributes
E1810 Practice for Evaluating Effects of Contaminants on Odor and Taste of Exposed Fish
E1879 Guide for Sensory Evaluation of Beverages Containing Alcohol
E1885 Test Method for Sensory Analysis--Triangle Test
E1958 Guide for Sensory Claim Substantiation
E2049 Guide for Quantitative Attribute Evaluation of Fragrance/Odors for Shampoos and Hair Conditioners by Trained Assessors
E2164 Test Method for Directional Difference Test
ISO StandardISO5495 Sensory Analysis--Methodology--Paired Comparison Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036.
ICS Number Code 67.240 (Sensory analysis)