Fayyad and Uthursamy (2002) have stated that the majority of the work (representing months or years) in creating a data warehouse is in cleaning up duplicates and resolving other anomalies. This article provides an overview of two methods for improving quality. The first is data cleaning for finding duplicates within files or across files. The second is edit/imputation for maintaining business rules and for filling in missing data. The fastest data-cleaning methods are suitable for files with hundreds of millions of records (Winkler, 1999b, 2003b). The fastest edit/imputation methods are suitable for files with millions of records (Winkler, 1999a, 2004b).