A method for cleaning data stored in a database, the method comprising providing a set of fixing rules. Each fixing rule incorporates a set of attribute values that capture an error in a plurality of semantically related attribute values, and a deterministic correction which is operable to replace one of the set of attribute values with a correct attribute value to correct the error. The method further comprises comparing at least two of the fixing rules with one another to check that the error correction carried out by one fixing rule is consistent with the error correction carried out by another fixing rule.