When compliance is poor it is easy to identify zones which require additional treatment (e.g., increase in phosphate dose). However, when compliance is good optimisation of treatment is more problematic since absence of failure is not absence of risk. The problem of assessing whether non-compliance in one zone or supply is significantly different from another was examined for the years 2006 to 2010 when the overall probability of infringing is already low (circa 0.01). Given the observed low probability of infringing the standard a Poisson model provides an appropriate model to characterise the variation of sample compliance in zones. For this case Pearson's chi-squared statistic provides the appropriate statistical test of significance viz.