Ngatia M, Pimental J. 2007. Comparisons of Organic Carbon Analyzers and Related
Importance to Water Quality Assessments. San Francisco Estuary and
Watershed Science 5. http://www.escholarship.org/uc/item/5h2074wx
This study tested whether analyzers using different methods were equally capable of measuring organic carbon in diverse environmental water samples from California’s Sacramento/San Joaquin Delta and its watersheds. The study also evaluated whether the different instruments might provide differing organic carbon concentration measurements, which could in turn trigger (or not) a regulatory requirement for enhanced coagulation at a water treatment plant. In Phase 1, samples were collected in eight monthly events at five stations associated with California’s State Water Project and analyzed using three high temperature combustion and three chemical oxidation instruments. Significant differences between instruments occurred in only 20% of the analyses. However, 80% of the observed differences were attributed to one combustion instrument that reported higher values compared to the other instruments. In Phase 2, four certified standards were analyzed with nine instruments. Results suggested that the main contributor of the observed differences was some instruments’ inability to remove inorganic carbon, an important step in the analytical process. There were no significant differences in the frequencies at which different instruments would have prescribed enhanced coagulation at a water treatment plant. We concluded that properly operating instruments using any of the standard methods were equally capable of analyzing the diverse concentration levels of organic carbon in the Delta.
Ngatia M, Gonzalez D, Julian SS, Conner A. 2010. Equivalence versus classical
statistical tests in water quality assessments. Journal of Environmental
Monitoring 12: 172-177.
To evaluate whether two unattended field organic carbon instruments could provide data comparable to laboratory-generated data, we needed a practical assessment. Null hypothesis statistical testing (NHST) is commonly utilized for such evaluations in environmental assessments, but researchers in other disciplines have identified weaknesses that may limit NHST's usefulness. For example, in NHST, large sample sizes change p-values and a statistically significant result can be obtained by merely increasing the sample size. In addition, p-values can indicate that observed results are statistically significantly different, but in reality the differences could be trivial in magnitude. Equivalence tests, on the other hand, allow the investigator to incorporate decision criteria that have practical relevance to the study. In this paper, we demonstrate the potential use of equivalence tests as an alternative to NHST. We first compare data between the two field instruments, and then compare the field instruments' data to laboratory-generated data using both NHST and equivalence tests. NHST indicated that the data between the two field instruments and the data between the field instruments and the laboratory were significantly different. Equivalence tests showed that the data were equivalent because they fell within a pre-determined equivalence interval based on our knowledge of laboratory precision. We conclude that equivalence tests provide more useful comparisons and interpretation of water quality data than NHST and should be more widely used in similar environmental assessments.