Fractional normalised variability vs. average count rate for all sources detected in each of the two epochs of the south (Field 1, upper panel) and north (Field 2, lower panel) XMM-Newton EPIC fields. These diagrams show the change of flux between two observations, which are about half a year apart. Sources with significant variability are marked with filled blue diamonds. Non-variable sources where the real difference in their count rates is smaller than their (quadratically added) uncertainty are marked with filled black circles. Dashed grey lines mark the zero and 100% variability levels. Marginal histograms show the distributions of average count rates and fractional normalised variability.