Correlations between relative abundances bear no relationship to the corresponding correlations between absolute abundances.

<p>(a) The pair of mRNAs labeled in red in <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1004075#pcbi.1004075.g002" target="_blank">Fig. 2</a>, shown on a linear scale. Values have been scaled and translated to have zero mean and unit variance. Upper panels show absolute abundances; the lower show relative abundances. The left panels show mRNA values over time; the right show the value of one mRNA plotted against the other at each time point. The correlation between the relative abundances is almost the complete opposite of that between the absolute abundances of this pair of mRNAs. (b) 2D histogram of the sample correlation coefficient observed for the relative abundances of a given pair of mRNAs, against the correlation observed for the absolute abundances of that same pair, over all pairs. The red and blue points correspond to the red and blue pairs of mRNA in <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1004075#pcbi.1004075.g002" target="_blank">Fig. 2</a>. White contour lines are shown at intervals of 100 counts. The top marginal histogram shows that the absolute abundances of most pairs are very strongly correlated. The right marginal histogram shows “the negative bias difficulty” [<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1004075#pcbi.1004075.ref004" target="_blank">4</a>].