A new tool for checks of data precision within the MRC British genetics of hypertension study by Caulfield, M, Papp, J, Pembroke, J, Munroe, P, Farrall, M, Dominiczak, A, Clayton, D, Ratcliffe, P, Lathrop, M, Webster, J, Keaveney, B, Benjamin, N, Brown, M, Samani, N, Connell, J