We have not investigated true masking with the data mining activity but checked whether there is an influence on the Bubble Point measurement before and after the product bacteria challenge test. We found that a Bubble Point shift only happened in 5 of 518 data sets and the fluids used in these 5 outliers were high foulant fluids. However, we do not know whether these fluids would have masked a flaw or not, as all filters used were integral pre- and post-use.