title: Fitting psychometric functions using a fixed-slope parameter: an advanced alternative for estimating odor thresholds with data generated by ASTM E679.
abstract: Psychometric functions are predominately used for estimating detection thresholds in vision and audition. However, the requirement of large data quantities for fitting psychometric functions (>30 replications) reduces their suitability in olfactory studies because olfactory response data are often limited (<4 replications) due to the susceptibility of human olfactory receptors to fatigue and adaptation. This article introduces a new method for fitting individual-judge psychometric functions to olfactory data obtained using the current standard protocol-American Society for Testing and Materials (ASTM) E679. The slope parameter of the individual-judge psychometric function is fixed to be the same as that of the group function; the same-shaped symmetrical sigmoid function is fitted only using the intercept. This study evaluated the proposed method by comparing it with 2 available methods. Comparison to conventional psychometric functions (fitted slope and intercept) indicated that the assumption of a fixed slope did not compromise precision of the threshold estimates. No systematic difference was obtained between the proposed method and the ASTM method in terms of group threshold estimates or threshold distributions, but there were changes in the rank, by threshold, of judges in the group. Overall, the fixed-slope psychometric function is recommended for obtaining relatively reliable individual threshold estimates when the quantity of data is limited.
authors.affiliation: School of Psychology, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand. m.hautus@auckland.ac.nz.
authors.forename: Mei
authors.initials: M
authors.lastname: Peng
authors.forename: Sara R
authors.initials: SR
authors.lastname: Jaeger
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Hautus
mesh_terms: D000328:Adult; D015331:Cohort Studies; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009812:Odorants; D011594:Psychometrics / Q000379:methods*; D012684:Sensory Thresholds / Q000502:physiology*; D055815:Young Adult
keywords: 3AFC; ASTM E679; method of limits; olfactory; psychometric function; threshold