[O3–05–04]: DEMENTIA CASE ASCERTAINMENT IN POPULATION‐BASED COHORT STUDIES: LESSONS FROM THE WHITEHALL II STUDY. (1st July 2017) - Ly, Amanda; Wilkinson, Tim; Schnier, Christian; Singh‐Manoux, Archana; Kivimaki, Mika; Brunner, Eric; Sanchez, Aida; Sudlow, Cathie; ,