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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.The 3-Class Attributes Plan is defined by recognizing three categories of quality, say good, marginal and bad. This device makes it possible to avoid calling an item simply acceptable or unacceptable as is done in the usual 2-Class Attributes Plan. The trivariate theory and some suggested forms of tabulating the plans are given. Two fields of potential application are proposed, one concerning hazardous substances in foods and the other amounts of active ingredient in dosage forms of drugs.