In cluster analysis, one does not start with any apriori notion of group characteristics. The definition of clusters emerges entirely from the cluster analysis - i.e. from the process of identifying "clumps" of objects. Clustering is used in many fields, including customer segmentation. An airline analyzing its customer data, for example, might find that there is a distinct cluster of passengers with the following characteristics: travel weekly, travel mainly one or two short-haul routes, book at the last minute, don't check bags.
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