Correlation of expression values pre-processed by two methods. Pearson correlation coefficients of expression measurements calculated by two pre-processing procedures are shown on the y-axis, probe sets ranked by average expression level over the four pre-processing methods are shown on the x-axis. Contours indicate equal density, as estimated using a Gaussian kernel density estimate, with kernel width optimised by leave-one-out maximum-likelihood. A. AML dataset. B. CNS dataset.