Moreover, the demographic profile of the catchment area population and the effects of subsequent differential attrition have led to an over-representation of more affluent groups and an under-representation of non-White minority ethnic groups compared with the national population. This may influence external validity of some study findings based on prevalence, although it should not influence adversely the longitudinal results provided the features affecting bias are included. An advantage of current analyses is the relatively new statistical techniques for taking account of missing data, and these are being used increasingly.45
