This step constitutes the core of the data preparation.
Data preparation steps may take 50 to 90 percent of the time and effort for the entire data mining process.
If you want good models you must have clean data.
The data you need may reside in multiple databases such as the customer database,
product database and transaction databases.
This means you need to integrate and consolidate the data into a single marketing database and reconcile differences in data values from the various sources.