A data warehouse is a large store of data that has been accumulated from a number of sources, and which is used to assist in the formulation of management decisions. The intent behind a data warehouse is to analyze data in order to assist in making decisions, rather than to process transactions. This is an improvement over trying to collect data from disparate databases, which may require a substantial amount of time. The result is broader company-wide access to key information, as well as a much faster response time than would normally be the case. The system can also be used to identify and clean any bad data that may have been imported from a supporting database, resulting in a high level of information quality.
Despite the obvious advantages of data warehouses, many companies have not found them to be cost-effective, usually because they were not properly assembled or maintained. This is a particular problem when the data are assembled from multiple databases that contain overlapping data that may not be consistent with each other. The most useful data warehouses are those that have been thoroughly verified. Data verification is especially necessary whenever a new database is added, since there would otherwise be a risk of having the newly-added data reduce the effectiveness of any data searches, calling into question the efficacy of the entire system.