Typically, the response rate in a direct marketing campaign is low. It is not unusual to see a response rate of less than 5%. As a result, the size of the “responder” class tends to be much smaller than the size of the “non-responder” class. Such situation where the class distribution is significantly skewed toward one of the classes is commonly known as the class imbalance problem [Jap00]. The more interesting class is usually the smaller class. Other examples of classification applications where class imbalance is common include the detection of oil spills in satellite images [KHM98], and the detection.
