More than 4,000 water samples were collected by the US Geological Survey (USGS) from 179 monitoring sites for the water-quality monitoring program at the Idaho National Engineering Laboratory from 1994 through 1995. Approximately 500 of the water samples were replicate or blank samples collected for the quality assurance/quality control program. Analyses were performed to determine the concentrations of major ions, nutrients, trace elements, gross radioactivity and radionuclides, total organic carbon, and volatile organic compounds in the samples. To evaluate the precision of field and laboratory methods, analytical results of the replicate pairs of samples were compared statistically for equivalence on the basis of the precision associated with each result. In all, the statistical comparison of the data indicated that 95% of the replicate pairs were equivalent. Within the major ion analyses, 97% were equivalent; nutrients, 88%; trace elements, 95%; gross radioactivity and radionuclides, 93%; and organic constituents, 98%. Ninety percent or more of the analytical results for each constituent were equivalent, except for nitrite, orthophosphate, phosphorus, aluminum, iron, strontium-90, and total organic carbon.