Duplicate split samples for internal quality control in routine water microbiology

In the microbiological examination of water, indicator organisms are sought by membrane filtration of 100-ml sub-samples of water. The same sample cannot be examined twice but the use of split samples, where two 100-ml sub-samples from the same batch are processed, has been explored. Binomial theory was used to determine the 95% confidence intervals for the results obtained. Two sets of 50 results were assessed by the method and the resulting control charts are illustrated.