Figure 3.

Normalization curve using spike-in controls. MA-plot with spike-in controls indicated. The blue points represent calibration controls where equal amounts of foreign RNA have been spiked into each sample. Red points represent ratio controls at three-fold and ten-fold differential expression. It can be seen that the spike-in probes are significantly offset from the gene probes and, therefore, could not be used for normalization purposes.

Oshlack et al. Genome Biology 2007 8:R2   doi:10.1186/gb-2007-8-1-r2
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