Evolution of the mean SN SALT2.4 stretch (x₁) as a function of redshift. Markers show the stretch plain mean (the error is estimated from the scatter) measured in redshift bins of equal sample size, indicated in light gray at the bottom of each redshift bin. Full and light markers are used when the fiducial or conservative samples are considered, respectively. The horizontal orange line represents the mean stretch for the nonevolving Gaussian model (last line of Table 3) on the fiducial sample. Best fits of our base drifting model are shown as blue, dashed blue, and gray when fitted on the fiducial sample, the conservative sample, or the SNfactory dataset only, respectively; all are compatible. The light blue band illustrates the amplitude of the error (including covariance) of the best-fit model when considering the fiducial dataset.