Jellyfish Compliance with SQIDEP · MCU15/0122

2
Technical Papers


 Table 1. Influent and effluent data for TSS, TP and TN.
 Event         TSS EMCi       TSS EMCe         TSS CRE        TP EMCi         TP EMCe                        TN EMCi        TN EMCe
                                                                                            TP CRE (%)                                     TN CRE (%)
 Number         (mg/L)         (mg/L)            (%)           (mg/L)          (mg/L)                         (mg/L)         (mg/L)
 1               180.0            13.0            93              0.2            0.2              0             1.3             1.3              0
 2               170.0            2.0             99              0.4            0.0             93             2.8             0.5             82
 3                19.0            2.0             89              0.1            0.1             44             1.1             0.5             55
 4                26.0            4.0             85              0.2            0.1             65             3.1             1.1             65
 5                25.0            4.0             84              0.2            0.1             29             1.9             1.9              0
 6                51.0            10.0            80              0.5            0.2             68             6.0             2.3             62
 7                74.0            8.0             89              0.3            0.1             74             3.2             1.6             50
 8                21.0            5.0             76              0.5            0.1             80             5.7             2.5             56
 9                13.0            1.0             92              0.2            0.1             63             2.0             1.3             35
 10               18.0            3.0             83             0.14            0.09            36             1.3             1.0             23
 Notes: EMCi: Event mean concentration of influent sample; EMCe: Event mean concentration of effluent sample; CRE: Concentration removal efficiency.

OUTLIER IDENTIFICATION                                                       Outlier
An outlier is defined as a data point that                                   Max Observation Below
is distinctly separate from and inconsistent                                 Upper Inner Fence
with the majority of the data points
(Walfish, 2006). Outliers can have negative
                                                                                Q3
effects on data analyses as they skew and
distort the data set or, conversely, can
provide useful information about abnormal
characteristics of the data (Seo, 2006).                                        Median
Therefore, it is a critical step to identify
                                                                                Q1
and detect outliers.

   A basic approach for identifying outliers                                 Min Observation Above
is outlined in Tukey (1977), and involves                                    Lower Inner Fence
simple numerical and graphical methods              Figure 1. Definition of features in a Box
that are used when constructing box plots           and Whisker Plot.
of the data (Walfish, 2006). It identifies a           As seen in the Box and Whisker Plots in
potential outlier as any data point more            Figure 2, a number of influent and effluent
than 1.5 interquartile ranges (IQRs) below
                                                    data points fell outside the inner fences,
the first quartile, or above the third
                                                    but were within the outer fences (see
quartile; that range is referred to as the
                                                    Tukey (1977) for calculation method of the
inner fences. An extreme outlier is a value
                                                    inner and outer fences). However, the data
that is outside the outer fences, and can be
                                                    between the inner and outer fences all
defined as three interquartile ranges below
                                                    fell within referenced parameters, which
the first quartile or above the third quartile
                                                    suggest typical concentration occurrences
(Tukey, 1977) and should be thoroughly
                                                    in Australia (Duncan, 1999); therefore, there
investigated. The box and whisker plots
                                                    is no justifiable reason to exclude any data
graphically display outliers, and examples
                                                    points from this data set. The CRE (%) data
can be seen in Figure 2 for the three
                                                    was also analysed for outliers, but all CRE
parameters of interest. Figure 1 describes
                                                    percentages fell within the inner fences.
the features of the box and whisker plot,
                                                    Therefore, all data points have been used.
including all relevant statistical measures.
                                                    NORMAL DISTRIBUTION
   There is no rigid mathematical
                                                    VALIDATION
definition of what constitutes an outlier in
the literature; it is a subjective decision. An     Following outlier evaluation, the next
analysis should be conducted to determine           step in this approach is to test for data
why the outlier may have occurred, and if a         normality using graphical methods
justifiable reason can be identified then the       and parametric statistical analysis.
data point may be legitimately excluded             The Stormwater Australia protocol calls
(Walfish, 2006). Most importantly, data             for log-transformed inlet and outlet-
that have been excluded as outliers must            paired samples (Stormwater Australia,
still be reported along with the reasons            2014), however if the baseline data
                                                                                                       Figure 2. Box and Whisker Plots for TSS,
for their exclusion, as they still form part        is already normally distributed then               TP & TN, with outliers labelled with their
of the evaluation process.                          there is no requirement to conduct a               event number.



WATER MAY 2015