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<正> Ⅰ. INTRODUCTION If there are s factors of which each factor has q levels in an experiment, where q>1, then the orthogonal array is often used. The number of experiments is 0(q~2) which is too large when q is comparatively large, for example, q≥9. The number of experiments may be decreased by BIB (balanced incomplete blocks) method for the case s=2 only (cf. [1]). Hence, it is necessary to find a method of which the number