Jellyfish Compliance with SQIDEP · MCU15/0122

                                                                                                                                           5
                                                                                                                     Technical Papers


mean CRE percentage for each pollutant,
                                                 Table 5. Confidence Levels for TSS, TP             Table 6. Minimum number of paired
rather than the influent and effluent            and TN about CRE % Removal.                        samples required to establish CRE%
concentration data. This is due to the                                                              at the 95% Confidence Level.
                                                                           TSS    TP     TN
fact that justification is required by the                                                                                 TSS   TP    TN
approving authority for the percentage           Sample Set Mean (µ)      91%    63%     51%
removal of the device, rather than the                                                              Z-Score (Zα/2)         1.96 1.96 1.96
                                                 Z-Score (Zα/2)           1.96   1.96    1.96
justification of influent and effluent                                                              COV                    0.07 0.44 0.54
concentrations at the testing site, which        Std. dev. (σ)            0.07   0.28    0.28
                                                                                                    % Removal              91% 63% 51%
can be justified by aligning with referenced     Number of Samples (n)     10     10     10
parameters for typical concentration                                                                Number of Paired
occurrences in Australia as described            Std Error of the Mean                              Samples Required
                                                                           4%    17%     17%                               0.03 3.46 9.87
in Duncan (1999). Table 5 presents               (SE)                                               to Meet 95%
the confidence intervals for the data.           Lower Confidence                                   Confidence Interval
                                                                          87%    46%     34%
                                                 Bound
   It can be observed that TSS has a much                                                       POPULATION VARIABILITY
lower range than TP and TN. This can be          Mean                     91%    63%     51%
                                                                                                & SAMPLE NUMBER
attributed to the fact that the TSS data is      Upper Confidence
more closely correlated and robust than                                   95%    80%     68%    It is clear from Table 6 that the number
                                                 Bound
the other two data sets, as a narrower                                                          of paired samples required to achieve
confidence interval will indicate a more        where                                           the 95% confidence level can vary greatly
precise estimate, while a wider confidence                                                      depending on the target pollutant and
interval indicates a less precise estimate                                                      the variability of the particular data set.
of the true population mean (Tan and Tan,         Next, setting the lower boundary              Also, it might appear strange that there
2010). From Table 5 it can be concluded         of the influent confidence interval to          is less than one paired sample required
that we are 95% confident that the true         the upper boundary of the effluent              to satisfy the 95% confidence level for
population mean of the per cent removal         confidence interval gives:                      the mean TSS removal figure (91%).
is between the lower and upper limit for                                                        Revisiting the TSS baseline data in
each pollutant type.                                                                            Table 1, however, shows a very consistent
                                                                                                per cent removal rate for each event
MINIMUM NUMBER
                                                                                                regardless of influent concentration.
OF PAIRED SAMPLES                                 The Coefficient of Variation (COV) is
Ensuring sufficient samples are collected       then substituted for in the above                 Contrast this, however, with a different
for any type of field research should be        equation using the standard relationship:       TSS data set, using more paired samples
considered imperative to ensure that                                                            than shown in Table 1, where the results
statistically relevant conclusions can be                                                       will be very different. Table 7 is a set of
derived from the results. However, in             It is more accurate to make the               TSS data from a different site, using a
most stormwater data sets, no statistical       assumption that                    as it is     different treatment device to that used
processes are utilised to guide the             highly likely that           . Substituting     for Table 1 data.
minimum sample size, and Burton and             and rearranging the above equations and
Pitt (2002) suggest that most follow                                                            Table 7. Alternative example
                                                solving for ‘n’ eventually yields:              influent and effluent data for TSS for
subjective professional judgement.                                                              a different site and different treatment
                                                                                                device to that used in Table 1.
  In order to numerically and statistically
show that there are sufficient data                                                             Event        TSS EMCi TSS EMCe     TSS
collected to ensure the required confidence                                                     Number        (mg/L)   (mg/L)     CRE (%)
                                                  This formula will then calculate the
level, Burton and Pitt (2002) proposed          minimum required number of paired               1               247        28         89
the use of a variety of basic equations,        samples for a specific confidence level.        2               300        280        7
based on coefficients of variation, Z-scores,   The results of this equation for the CRE
sample set means, and standard deviations.                                                      3               233        113        52
                                                test data can be seen in Table 6.
                                                                                                4                21        16         24
   The Urban Stormwater BMP                        It is shown in Table 6 that TSS requires
Performance Monitoring Manual                                                                   5                32        23         28
                                                less than one paired sample, TP requires
(Geosyntech, 2009), however, describes          just over three paired samples, and TN          6                19        14         26
an equation specifically for sampling           requires almost 10 paired samples to            7                55        40         27
stormwater treatment removal rates              ensure a 95% confidence level of the
                                                                                                8                27         8         70
around a stormwater treatment device,           mean % reductions in concentrations
based on similar methods described              from Table 5. Given that there are              9                67        10         85
in Burton and Pitt (2002). The BMP              10 samples in the data set, and the             10               28        20         29
Performance Monitoring Manual                   minimum number of paired samples
                                                                                                11               70        35         50
(2009) begins with the formula defining         required for each pollutant is all less
percentage removal (as a fraction               than 10, we conclude this data set will         12               45        14         69
rather than a per cent):                        achieve statistical significance with a 95%     13              208        156        25
                                                confidence level. Table 6 is a calculation      14              121        114        6
                                                used to inform required sample numbers,
                                                                                                15               38        28         26
                                                rather than an a posteriori calculation.



                                                                                                                          MAY 2015 WATER