Measurement uncertainty of the c parameter in the SNLS survey as a function of redshift, for events spectroscopically identified. Solid circles show the contribution of the source shot noise alone, and the squares include intrinsic fluctuations from event to event (also called colour smearing). At z> 0.7, the shot noise contribution becomes essentially constant because the colour measurement relies on bluer and bluer restframe bands, which are more and more sensitive to colour changes. This might look favourable, but accounting for intrinsic fluctuations from event to event (squares), very large in the UV, swamps this benefit. (Data obtained from fitting light curves from Guy et al. 2010.)