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Significance and Use
Magnitude estimation may be used to measure and compare the intensities of attributes of a wide variety of products.
Magnitude estimation provides a large degree of flexibility for both the experimenter and the assessor. Once trained in magnitude estimation, assessors are generally able to apply their skill to a wide variety of sample types and attributes, with minimal additional training.
Magnitude estimation is not as susceptible to end-effects as interval scaling techniques. These can occur when assessors are not familiar with the entire range of sensations being presented. Under these circumstances, assessors may assign an early sample to a category which is too close to one end of the scale. Subsequently, they may “run out of scale” and be forced to assign perceptually different samples to the same category. This should not occur with magnitude estimation, as, in theory, there are an infinite number of categories.
Magnitude estimation is one frequently used technique that permits the representation of data in terms of Stevens' Power Law.
The disadvantages of magnitude estimation arise primarily from the requirements of the data analysis.
Permitting each assessor to choose a different numerical scale may produce significant assessor effects. This disadvantage can be overcome in a number of ways, as follows. The experimenter must choose the approach most appropriate for the circumstances.
Experiments can be designed such that analysis of variance can be used to remove the assessor effects and interactions.
Alternatively, assessors can be forced to a common scale, either by training or by use of external reference samples with assigned values (modulus).
Finally, each assessor's data can be brought to a common scale by one of a variety of normalizing methods.
Logarithms must be applied before carrying out data analysis. This becomes problematic if values are near threshold, as a logarithm of zero cannot be taken (see 11.2.1).
Magnitude estimation should be used:
When end-effects are a concern, for example when assessors are not familiar with the entire range of sensations being presented.
When Stevens' Power Law is to be applied to the data.
Generally, in central location testing with assessors trained in the technique. It is not appropriate for home use or mall intercept testing with consumers.
This test method is only meant to be used with assessors who are specifically trained in magnitude estimation. Do not use this method with untrained assessors or untrained consumers.
1.1 This test method describes a procedure for the application of unipolar magnitude estimation to the evaluation of the magnitude of sensory attributes. The test method covers procedures for the training of assessors to produce magnitude estimations and statistical evaluation of the estimations.
1.2 Magnitude estimation is a psychophysical scaling technique in which assessors assign numeric values to the magnitude of an attribute. The only constraint placed upon the assessor is that the values assigned should conform to a ratio principle. For example, if the attribute seems twice as strong in sample B when compared to sample A, sample B should receive a value which is twice the value assigned to sample A.
1.3 The intensity of attributes such as pleasantness, sweetness, saltiness or softness can be evaluated using magnitude estimation.
1.4 Magnitude estimation may provide advantages over other scaling methods, particularly when the number of assessors and the time available for training are limited. With approximately 1 h of training, a panel of 15 to 20 naive individuals can produce data of adequate precision and reproducibility. Any additional training that may be required to ensure that the assessors can properly identify the attribute being evaluated is beyond the scope of this test method.
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
E1871 Guide for Serving Protocol for Sensory Evaluation of Foods and Beverages
ISO StandardsISO8589:1988 Sensory Analysis--General Guidance for the Design of Test Rooms
ICS Number Code 67.240 (Sensory analysis)