Histograms of signal ratio values. A and D represent histograms of the Test over Control ratio (T/C) values. B and E represent histograms of the logarithm of the Test over Control ratio (Log(T/C) values. C and F represent histograms of the Control over Test ratio (C/T) values. A, B and C histograms were drawn with ratio values of probes where the gene was absent in the test sample. D, E and F histograms were drawn with ratio values of probes where the gene was present in the test sample. Red lines are best fits of a Normal distribution to the drawn histogram. Blue lines are best fits of the Gamma distribution to the corresponding histograms. Due to the negative values of some Log (T/C) ratios, it was not possible to fit a Gamma distribution to the histograms in B and E. All the histograms where based on one Streptococcus pneumoniae microarray using the mix control.