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. This content downloaded from 128.83.205.78 on Thu, 18 Jun 2015 16:13:08 UTC All use subject to JSTOR Terms and Conditions Hedayat and John (1974) studied the

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... e of resistance of a balanced incomplete block design (BIBD) to the removal of one treatment. They showed that a necessary and sufficient condition for a BIBD to be resistant (that is, retain its variance balance) upon removal of a single treatment is that the two proper subdesigns created by this removal be BIBDs. Herein their results on the structure of resistant BIBDs are generalized and the relationship of such designs to t-designs (which are of interest in combinatorial theory) is shown. The main result is a structure theorem which states that a BIBD in v treatments and k plots per block is resistant to the removal of any subset of a specific set of n treatments, n < k < v, if and only if all of the 2n proper subdesigns formed when all n treatments are removed are BIBDs in all vn remaining treatments. This leads to a lower bound (of 2A(vn)) on the number of blocks required for fully locally resistant BIBDs which are not essentially trivial. Examples of resistant designs are given.