Caboolture Region Water Quality Study - Moreton Bay Regional ...

Caboolture Water Quality Study
Appendix B: Data presentation- Box and Whisker plots
Box and whisker plots are a widely accepted way of presenting water quality data (Figure 17). They show
where the data points are concentrated (the box) and the outlying values (the whiskers, asterisks and
circles). Box and whisker plots are therefore a useful way of visually presenting the distribution of the data.
Box and whisker plots are used in this report to assess compliance with guidelines. For easy comparison
with the guidelines (derived from the 20 th and 80 percentiles of reference values for most parameters) the
boxes represent the 20 th and 80 th percentiles. The top of the box in these plots is the 80 th percentile (80
percent of the data fall below this line), while the bottom of the box represents the 20 th percentile (20
percent of the data fall below this line). The dot in the middle of the box represents the median (50 percent
of the data fall above and 50 percent below this number).
The whiskers on the box extend to data points that are up to 1½ times the Inter Quartile Range (IQR). The
IQR is defined as the difference between the 75 th and the 25 th percentiles, and is equal to the range of
about half the data. Outliers which are less than three times the IQR are shown as asterisks, while those
greater than three times the IQR are shown as circles. The statistical and graphical package S-Plus 4.5
was used to produce all box plots and calculate percentiles.
Values within 3 times the Inter
Quartile Range
80th percentile: 80 percent of
values fall below this line
Median
20th percentile: 20 percent of
values fall below this line
Whiskers: values within 1½
times the Inter Quartile Range
Values greater than 3 times
the Inter Quartile Range
Figure 18. Schematic representation of box and whisker plots.
Box and whisker plots are used in this report to assess compliance of each indicator at each site with
the appropriate guidelines. The data for one or more waterways are presented together with the
reference values for those watertypes (Figure 18). For easy comparison with the reference values, the
boxes are colour coded according to their watertype (Figure 18, Table 21). The label for the box for
each site is also colour coded according to compliance with the guidelines. If the site complied with the
guidelines for that indicator, the site label is printed in black; if the site did not comply the label is
printed in red. The number of data points in both the site and reference data is also recorded below
each box.
Figure 1k Chlorophyll-a (g/L) at sites in an example waterway
20
Chlorophyll-a (g/L)
15
10
5
0
(N=10) (N=12) (N=12) (N=12) (N=12) (N=12) (N=12) (N=49) (N=57) (N=336) (N=110) (N=111) (N=169) (N=54)
SITE-1 SITE-2 SITE-3 SITE-4 SITE-5 SITE-6 SITE-7 O OC ECLE ME UE LC UC
Monitoring Sites
Reference Sites
Figure 19. Example of a box and whisker plot, showing values recorded at monitoring sites (left) and reference sites
(right) for chlorophyll-a. Watertypes are colour-coded, and descriptions for the watertype-code are presented in
Table 22. Note that sites 1, 2, and 6 complied with water quality guidelines, whereas sites 3, 4, 5 and 7 exceeded
guidelines.
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