Input variability. Histogram for variability in TFBS length in our input data from Transfac Public database. Population standard deviation and mean have been counted for each set of TFBS. The x-axis shows the ratio of SD and mean which can be seen as a measure of variability in data. Each TF and all its BSs are assigned into one bin in x-axis. The y-axis shows the fraction of total TF or BS population that fell into each bin in x-axis.