1. A method of computing a similarity between a first transaction having a set of properties and a second transaction having the set of properties, the method comprising: computing an initial weight for each of the properties of the set of properties; computing a similarity between each of the properties of the first transaction and the properties of the second transaction; adjusting the initial weight for each of the properties based on a measure of the commonness of each of the properties of the set of properties; normalizing the adjusted weights; and computing the similarity by summing the products of the normalized adjusted weights and the computed similarities.