Industries such as textiles, rubber, and chemicals, whose products must possess certain chemical or physical qualities, sometimes find it economical to apply different combinations of basic materials and still achieve a perfect product In cotton fabrics a change in the mix of cotton from different parts of the world may reduce cost and improve profits. In many cases, a new mix is accompanied by either a favorable or an unfavorable yield of the final product, making it difficult to judge correctly the origin of the variances. A favorable mix variance, for instance, may be offset by an unfavorable yield variance, or vice versa.