Effects of compensating the threshold parameterγ. (a Histograms of the logarithms of human cDNA microarray data  (from the Stanford Microarray Database , ID 5731). The histograms for the original intensity data (blue) and local background-subtracted  data (green) both skew to the left, whereas that for the γ-subtracted data shows the typical shape of a normal distribution (red). ƒ, frequency of data. (b) Normal probability plots for logarithms of the original intensity data (blue), local background-subtracted data (green), and the γ-subtracted data (red). In the plot, x-axis presents theoretical values for normal order statistic medians, whereas y-axis presents ordered logarithms of data . The γ-subtracted data show reasonable linearity, characteristic of a normal distribution. Definitions of parameters and data treatments are described in Materials and Methods.