AS 2542.2.4:2014 Sensory analysis
6.2 Number of assessors
Choose the number of assessors so as to obtain the sensitivity required for the test (see discussion in A.3). Using large numbers of assessors increases the likelihood of detecting small differences between the products. However, in practice, the number of assessors often is determined by material conditions (e.g. duration of the experiment, number of available assessors, quantity of product). When testing for a difference, typical numbers of assessors are between 32 and 36. When testing for no meaningful difference (i.e. similarity), twice as many assessors (i.e. approximately 72) are needed for equivalent sensitivity.
Avoid replicate evaluations by the same assessor whenever possible. However, if replicate evaluations are needed to produce a sufficient number of total evaluations, every effort should be made to have each assessor perform the same number of replicate evaluations. For example, if only twelve assessors are available, have each assessor evaluate three triads to obtain a total of 36 evaluations.
NOTE Treating three evaluations performed by twelve assessors as 36 independent evaluations is not valid when testing for similarity using Table A.2. However, the test for difference using Table A.1 is valid even when replicate evaluations are performed (see [9] and [10]). Recent publications (see [7] and [8]) on replicated discrimination tests suggest alternative approaches for analysing replicated evaluations in discrimination tests.
7.2 Present the three samples of each triad simultaneously if possible, following the same spatial arrangement for each assessor (e.g. on a line to be sampled always from left to right, in a triangular array). Within the triad, assessors are generally allowed to make repeated evaluations of each sample as desired (if, of course, the nature of the product allows for repeated evaluations).
7.3 Instruct the assessors to evaluate the reference sample first, then evaluate the two coded samples in the order in which they were presented. Inform the assessors that one of the coded samples is the same as the reference and that one is different from the reference. Instruct the assessors to indicate either which of the two coded samples is the same as the reference, or which of the two coded samples is different from the reference.