Introduction: In many applications, users may not be interested in having a single class (or concept) described or characterized, but rather would prefer to mine a description that compares or distinguishes one class (or concept) from other comparable classes (or concepts).Class discrimination or comparison (hereafter referred to as class comparison) mines descriptions that distinguish a target class from its contrasting classes. Notice that the target and contrasting classes must be comparable in the sense that they share similar dimensions and attributes. For example, the three classes, person, address, and item, are not comparable.
However, the sales in the last three years are comparable classes, and so are computer science students versus physics students. Our discussions on class characterization in the previous sections handle multilevel data summarization and characterization in a single class. The techniques developed can be extended to handle class comparison across several comparable classes. For example, the attribute generalization process described for class characterization can be modified so that the generalization is performed synchronously among all the classes compared. This allows the attributes in all of the classes to be generalized to the same levels of abstraction. Suppose, for instance, that we are given the All Electronics data for sales in 2003 and sales in 2004 and would like to compare these two classes. Consider the dimension location with abstractions at the city, province or state, and country levels. Each class of data should be generalized to the same location level. That is, they are synchronously all generalized to either the city level, or the province or state level, or the country level. Ideally, this is more useful than comparing, say, the sales in Vancouver in 2003 with the sales in the United States in 2004 (i.e., where each set of sales data is generalized to a different level). The users, however, should have the option to overwrite such an automated, synchronous comparison
with their own choices, when preferred.
“How is class comparison performed?” In general, the procedure is as follows:
1. Data collection: The set of relevant data in the database is collected by query processing and is partitioned respectively into a target class and one or a set of contrasting class(es).
2. Dimension relevance analysis: If there are many dimensions, then dimension relevance analysis should be performed on these classes to select only the highly relevant dimensions for further analysis. Correlation or entropy-based measures can be used for this step (Chapter 2).
3. Synchronous generalization: Generalization is performed on the target class to the level controlled by a user- or expert-specified dimension threshold, which results in a prime target class relation. The concepts in the contrasting class(es) are generalized to the same level as those in the prime target class relation, forming the prime contrasting class(es) relation.
4. Presentation of the derived comparison: The resulting class comparison description can be visualized in the form of tables, graphs, and rules. This presentation usually includes a “contrasting” measure such as count% (percentage count) that reflects the comparison between the target and contrasting classes. The user can adjust the comparison description by applying drill-down, roll-up, and other OLAP operations to the target and contrasting classes, as desired.
The above discussion outlines a general algorithm for mining comparisons in databases. In comparison with characterization, the above algorithm involves synchronous generalization of the target class with the contrasting classes, so that classes are simultaneously
