A survey of distance/similarity measures for categorical data

Similarity or distance between two objects plays a fundamental role in many data mining tasks like classification and clustering. Categorical data, unlike numeric data, conceptually is deficient of default ordering relations on the attribute values. This makes the task of devising similarity or distance metrics and data mining tasks such as classification and clustering of categorical data more challenging. In this paper we formulate a taxonomy of various distance or similarity measures used in conjunction with data whose attributes are categorical. We categorize the existing measures into two broad classes, namely, Context-free and Context-sensitive measures for categorical data. In addition, we suggest a taxonomy of the clustering approaches for categorical data. We also propose a hybrid approach for measuring similarity between objects. We make a relative comparison of the strengths and weaknesses of some of the similarity measures and point out future research directions.