Odour Threshold Investigation 2012 - Bay of Plenty Regional Council

Appendix 3 – Dr Ray Littler analysis report
Brief report on H 2 S Detection Threshold Study
Ray Littler
Waikato Applied Statistics Unit
University of Waikato
The main statistical task is to characterise the distribution of the 1371 H 2 S detection
threshold results collected from 59 subjects, residents of either Rotorua or Whakatane. We
are particularly interested in whether the fact that Rotorua residents are accustomed to
higher ambient H 2 S levels affects their detection thresholds in comparison to those for
Whakatane residents. Untrained panellists were used as subjects because of the context of
the study, and tests were structured so that subjects evaluated samples from a dilution series
repeatedly at each of three test sessions. Subsidiary questions relate to the repeatability of
test results, and the possible effects of some demographic variables on the thresholds.
Thresholds determined for n-butanol in the study have also been analysed.
Data aggregation
Because of the dilution series nature of the study, the raw results are necessarily rather
discrete (see Figure 1), and it is sensible to take as the primary unit for analysis the mean
threshold per session per panellist. In addition the steps between possible values are wider
for high thresholds, so we may find absolute measurement error greater for high threshold
values.
Because of the number of tests per session we expect session means to have approximately
continuous distributions. So most of our analyses are actually based on either
Panellist.Session means or Panellist means. The Panellist.Session means are averaged
over about eight individual test results (each result the outcome of a series of dilution
responses).
Environmental Publication 2012/06–A review of odour properties of H 2 S Odour Threshold Investigation 2012 61