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This paper presents an improved test data compression scheme based on a combination of test data compatibility and dictionary for multi-scan designs to reduce test data volume and thus test cost. The proposed method includes two steps. First a drive bit matrix with less columns is generated by the compatibilities between the columns of the initial scan bit matrix, also the inverse compatibilities and the logic dependencies between the columns of mid bit matrixes. Secondly a dictionary bit matrix with limited rows is constructed, which has the properties that for each row of the drive bit matrix, a compatible row exists or can be generated by XOR operation of multiple rows in the dictionary bit matrix and the total number of rows used to compute all compatible rows is minimal. The rows in the dictionary matrix are encoded to further reduce the number of ATE channels and test data volume. Experimental results for the large ISCAS 89 benchmarks show that the proposed method significantly reduces test data volume for multi-scan designs.