Replicate data are more similar to each other after local normalization. The ratios of signal ratios from corresponding pairs of replicate spots on the same array were determined for all array elements (Ratio 1 / Ratio 2). Increased agreement between replicate measurements leads to ratios of signal ratios closer to 1. The data are from the same array as in Figure 2 (left side) and Figure 3. Left histogram: replicate data distribution before normalization; right histogram: replicate data distribution after normalization.