Plot of the standardised log intensity of 1X, 3X, 4X and 5X against 2X. The signal intensities are based on the average of two replicates across 302 single nucleotide polymorphisms that map to the X chromosome using National Center for Biotechnology Information Build 33. Figure 1b plots log (copy number) as a function of estimated log (intensity ratio) (C). The black dots indicate different samples (1X to 5X). The red line is the linear regression result using log (copy number) as the response and estimated log intensity ratio as the predictor. The blue lines indicate the 95 per cent confidence interval for the response, ie the natural log of the copy number.