This guide is a short introduction to the imputation flags in the 2001 (ME01) and 2011 (ME11) members data files of the Office for National Statistics Longitudinal Study (ONS-LS). ONS has developed several methods that have been applied to resolve item non-response, invalid responses, and item inconsistencies in the 2001 and 2011 Census data. These methods included deterministic editing and imputation.