Some countries use forms of random rounding as a post-tabular method to protect Census frequency counts disseminated in tables. These methods typically result in inconsistencies between aggregated internal cells to marginal totals and across same cells in different tables. A post-tabular method for perturbing frequency counts is proposed which preserves totals and corrects to a large extent inconsistencies. The perturbation is based on invariant probability transition matrices and the use of microdata keys. This method will be compared to common pre and post-tabular methods for protecting Census frequency counts.