Representation of likelihood as function of heavy-atom contribution to the structure factor for one derivative. The panels on the left show three possible choices of the heavy-atom contribution. Since a change in the heavy-atom contribution shifts the centre of the probability distribution for the derivative amplitude (see Fig. 2), it changes the amount of overlap of the distributions from different crystals. The combined distribution from the native and two derivative amplitudes (multiplied by the prior probability of the true structure factor, as in equation 7) is shown as a surface plot in the three middle panels. The likelihood is the volume under this surface and the panel on the right shows the likelihood as a function of the size of the heavy-atom contribution, with dots highlighting the values corresponding to the three pairs of plots to the left. An animated version of this figure is available as supplementary material1.