Water quality control trials: statistical tables for direct comparison between membrane filtration bacterial counts...

Experiments in the quality control of water samples are being conducted in the Public Health Laboratory Service and the water industry in the United Kingdom. The number of distributions which have been made is 7 and 92 laboratories are now participating. The methods used for preparing and distributing samples are described. Some participating laboratories use the multiple tube method and some use membrane filtration to assess the presence of coliforms and Escherichia coli. The results are, therefore, a mixture of estimated numbers and direct colony counts. In order to compare results from these two different laboratory methods statistical tables have been compiled to show the most likely multiple tube result corresponding to each colony count. Tables relating to two commonly used tenfold dilution series are presented. To illustrate how these tables may be used we present results from a typical quality control distribution. The analyses of these results are generally satisfactory but show a tendency for lower counts using the membrane filtration method and more false negative results with E. coli counts.