Distributions of the best-fit parameters of the fourth-order polynomials used for modelling the cross-calibration bias of single clusters for the MOS1/pn pair shown with the red histogram. The best-fit Gaussians to the above distributions are shown as a dashed blue line. The best-fit centroid of the Gaussian and the central interval containing 68% of the probability interval are indicated with a dashed black line and the yellow band, respectively.