Commentary.

This is an informative article, which describes a clinical trial, conducted using multiple data sources, which provide better case ascertainment than single sources, but less complete ascertainment than community based surveys with cognitive screening. Although studies of young onset dementia differ in methodology and may be based on small numbers which produce less stable estimates, a community based survey in Finland, that included only cases of severe dementia and excluded associated depression reported 4.8 times higher prevalence for all cause dementias in people aged 30-64 years.