At that point, the deviation may come as much from inaccuracies in the present determination as from inaccuracies in the reference values, the latter derived with the traditional single-marker formula. In Figure we plot the coefficient of variation (standard deviation over average) of the mobility of each compound over the six replicates (again, three at each pressure). The spread in migration times is high, which is to be expected from running each half of the replicates at different pressures. The conversion to mobilities, even with the single marker formula and no ramp correction reduces the variability to little more than 2%. This does not change with the addition of the ramp correction, meaning that the large deviation in Figure is caused by a systematic shift, as one would expect. But the addition of a second marker reduces the variability further to less than 0.5% between the six runs at different pressures. In this light, the deviations of ∼ 2% of the two-marker formulas in Figure are most likely due to variability in the original determination. These new mobility values for the chosen standards are available in the supplementary material. Notice still that this variability happens within the same set of experiences, and somewhat higher variability should be expected in inter-laboratory comparisons.