Contribution of various sources to correlated uncertainties, averaged over sliding Δz = 0.2 bins for the SNLS3 analysis (data from Guy et al. 2010). “Colour smearing” refers to the effect of uncertainties of the band-dependent residual scatter model (see Sect. 3.3). The steep increase at high redshift of this contribution and of that from SN model training statistics are both due to those events being measured in bands bluer in the rest-frame than the lower redshift events. We note that these two contributions are indeed going down with sample size.