A method for determining the presence of a copy number imbalance in genomic DNA of a test sample is provided. The method can separately measure hybridization of a single test sample to a first hybridization array and hybridization of a plurality of reference samples to a plurality of other, respective test arrays. A determination of copy number can be based on the best fit reference array, relative to the test array. The best fit can be determined based on the closest or most similar signal-to-noise ratio of the measured signals.