The continuing improvement in performance has presented an opportunity to re-optimise treatment. However, with the resulting higher level of compliance, circa 99%, comes a proportionately higher random variability which makes it difficult to determine significant differences in compliance. The analysis has shown that some resolution can be done with large data sets (equivalent to 30 years of regulatory reporting). Comparison of the observed zonal compliance distribution with a purely random (Poisson) distribution showed the distribution was found to be a zero inflated overdispersed Poisson with outliers.