Odour Threshold Investigation 2012 - Bay of Plenty Regional Council

Figure 7.11
Log 10 mean session data for the entire group and locations.
Box-Cox transformations 77 were also investigated and similarly showed
improvements in the normality of the resulting dataset. A lambda value of -0.15 was
derived which is positioned between the natural log and reciprocal square root
transformation families. For the analysis in this project the log-transformed data
above has been chosen due to this supporting output from several different
statistical methods of transforming datasets.
7.5.4 Mean H 2 S threshold calculations for the dataset
Table 7.1 show a statistical summary of the dataset grouped by location as well as
the entire dataset. The geometric mean was determined by taking the logarithms of
the original data values, calculating the mean of the log values, and then converting
that back to a non-log form (i.e. exponentiating or back-transforming e.g. 10 x ). As
expected for this type of skewed data, the geometric mean is approximately equal to
the median value.
Table 7.1
Mean threshold values (µg/m 3 ) for all data and by location.
Statistic
summary
All data
(µg/m 3 )
Rotorua
(µg/m 3 )
Whakatāne
(µg/m 3 )
Mean 0.99 1.39 0.73
Median 0.67 1.12 0.49
Geometric mean 0.71 1.11 0.53
77 Osborne, J.W., 2010, Improving your data transformations: Applying the Box-Cox transformation,
Practical Assessment, Research & Evaluation, Volume 15, Number 12.
http://pareonline.net/pdf/v15n12.pdf
42 Environmental Publication 2012/06–A review of odour properties of H 2 S Odour Threshold Investigation 2012