Extremes of baseline cognitive function determine the severity of delirium: a population study by Tsui, A, Yeo, N, Searle, SD, Bowden, H, Hoffmann, K, Hornby, J, Goslett, A, Weston-Clarke, M, Lanham, D, Hogan, P, Seeley, A, Rawle, M, Chaturvedi, N, Sampson, EL, Rockwood, K, Cunningham, C, Ely, EW, Richardson, SJ, Brayne, C, Muniz Terrera, G, Tieges, Z, MacLullich, AMJ, Davis, D