High strength events
In this section we present ROC curves obtained for high
strength events, and for both chlorine and turbidity.
Performance evaluation for chlorine
Table 3 presents the ROC for the chlorine measurement and
shows that at a 5% FPR, OBCPD provides the highest TPR
value compared to the other change-point detection algorithms.
Also, theresult obtained withOBCPDshowsaTPRequal to97%
for a contamination strength higher than or equal to 7. We
observe that the performances of the four algorithms improve
as we increase the strength of the contamination. Figure 7 presents
the ROC curves corresponding to the cases where the
contamination strength is equal to 5 (left) and 8 (right).
Performance evaluation for turbidity
Similarly to Table 3, for the turbidity measurement we
observe that at a 5% FPR, the OBCPD provides the highest
TPR value compared with the other change-point detection
algorithms. Figure 8 presents the ROC curves corresponding
to the cases where the contamination strength is equal to 5
(left) and 8 (right). ROC results for turbidity are generally
lower than for chlorine, especially for SPRT and CUSUM.
This is due to two change-points being identified for each
event in the lower noise data as discussed in the ‘Results’
section. There, the second change-point (end event) is
counted as a false positive.
Low strength events
Here, we focus on the cases where the contamination
strength is lower than or equal to 1, and for both chlorine
and turbidity.