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It is paradoxical that complaints about water quality are usually based on subjective properties, that is, odor, taste and appearance, but analytical emphasis is usually chemical, physical, or biological. The reason for this anomaly is that the latter properties are more readily standardized, and values may be reported within relatively narrow numerical limits. Subjective properties must be measured by persons. Since people vary in sensory acuity, it is much more difficult to establish either intensity or characterization values for subjective properties. Although one often sees threshold odor or taste data presented in absolute values, there should be cognizance of the implied uncertainty of such values. Individuals vary in sensory ability from day to day or within a day. Groups of individuals show even greater tendency to vary. Consequently, subjective data are best described in a statistical or probability manner. An average or median value plus the range or associated confidence limits tell much about the distribution of a sensory property. Sensory testing of water is generally restricted to taste and odor, color and appearance. Taste and odor are usually combined as a single term in water technology, probably because consumer complaints on these properties are often difficult to separate. Most recent published information actually concerns odor and says very little about taste. Much more needs to be learned about both properties. Color may be rated against fixed standards, but often color or appearance judgments are recorded as part of the analytical summary of water quality. In the following section, various aspects of sensory testing will be examined.