The average within-slide variance (× 10-2) of log ratios M of control spots is shown after applying different normalization schemes. The three types of control spots derived from genomic DNA were used: salmon sperm (SS) DNA; Cot-1 DNA; rice DNA. Their intensities were above background as a result of nonspecific cross-hybridization. The locations of the replicated control spots were spatially distributed across the array. Comparison of corresponding log ratios M thus provides a measure for the spatial consistency of results produced by normalization.