Odor Threshold Determination – A review of the effect of number of assessors

Over 10 years ago the European standard test method for determination of odor concentrations (EN13725) made significant advancements in the field of olfactometry. Steps were taken to standardize the olfactometer and the assessors. Testing with standard odorant n-butanol allowed laboratories to select assessors and continually monitor their performance with a universally agreed upon metric. Standardized assessors led to the agreement that environmental odor threshold tests could be determined with a sample of four assessors completing two testing rounds (eight responses). ASTM International Standard E679 was revised in 2004 to acknowledge these advancements.Since the adoption of these standards, the question is often raised if four assessors provide sufficient precision for determination of an odor threshold value for environmental odor samples. This topic has been part of discussions at recent CEN Working Group meetings for the review and revision of EN13725.A new study conducted by St. Croix Sensory has reviewed multiple detection threshold calculations determined with triangular forced choice testing utilizing n=1 to n=10 assessors. Statistical analysis was completed for all possible combinations of assessor responses for each sample. This paper will discuss detailed results of the analysis and provide recommendations for number of assessors needed for specific odor study objectives, e.g. source comparisons, routine testing, performance testing, etc.