Since the inception of the U.S. Geological Survey National Field Quality Assurance Program, over 85,000 proficiency samples have been analyzed by water-quality analysts. This includes more than 10,000 alkalinity samples, more than 15,000 pH samples, and more than 16,000 specific conductance samples, which were analyzed from 1990 through 1997, and a total of more than 43,000 proficiency samples analyzed from 1979 through 1989. The analyte values were evaluated to determine the fourth-spread, a measure of the width of the middle half of the data, and the F-pseudosigma, a robust replacement for the standard deviation, for each of the different measurement ranges. The result of the statistical evaluation showed that the vast majority of reference sample measurements made by water-quality analysts were within acceptable ranges. From 1990 to 1997, the measurement of pH had the highest level of acceptable results, 98.4 percent, followed by specific conductance with 95.2 percent acceptable results, and alkalinity with 88.6 percent acceptable results. The statistical summary of pH indicates the calculated fourth-spread values for the entire tested range is +0.06 pH units. For specific conductance, the magnitude of the fourth-spread increases as the magnitude of the specific conductance ranges increases. The average relative fourth-spread percent for all reported specific conductance values is +1.8 percent. From 1990 through 1997, the evaluation of the results for alkalinity measurement for the average fourth-spread was determined to be + 3.3 milligrams per liter as calcium carbonate.