Odour Threshold Investigation 2012 - Bay of Plenty Regional Council

Figure 4 Untransformed is non-normal, Figure 5 Transformed is-normal, uniform
increasing SD
SD
Is there a trend over sessions?
It would appear plausible that panellists might become better at detecting H 2 S over three
sessions leading to a negative trend in threshold. Working in the log scale we show in
Figure 6 (using the trimmed data) that we can readily “see” a trend over the three sessions
for many individual panellists. However a number of them are in the “wrong” direction. A
formal non-parametric test gives some support to the existence of a reduction in mean
threshold over sessions.
Figure 6 also gives some guidance as to the between-session variation in threshold for a
subject, whether it be due to trend or “random” variation. A more careful analysis of the
variation between sessions provides the following prediction. If we collect untransformed
mean threshold values for a subject for three sessions, then we expect the ratio of the
maximum value to the minimum (of the three) to be about 2.78. (This is based on an
estimate of the between-session standard deviation for log 10 (mean threshold) of 0.283).
Environmental Publication 2012/06–A review of odour properties of H 2 S Odour Threshold Investigation 2012 65