It is clear that in all three datasets shown in Figure , the maximum evidence is for model 3. However, as the number of contrasts investigated is reduced, there is a change in the relative evidence between the models. This is particularly notable when a dataset containing a single isotopic contrast is compared with those with three or five datasets. The reduction may be rationalised as when less experimental data is present, in particular when contrast variation is not used to pick out specific components of the system, a model is less able to pick out specific elements of the structure. This agrees well with perceived wisdom regarding neutron reflectometry measurements .