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Evaluating the Rarity of Handwriting Formations

Identifying unusual or unique characteristics of an observed sample in useful in forensics in general and handwriting analysis in particular. Rarity is formulated as the probability of letter formations characterized by a set of features. Modeling the distribution as a probabilistic graphical model several probabilities are inferred: the probability of random correspondence (PRC) as a measure of the discriminatory power of the characteristics, conditional PRC associated with a given sample and the probability of finding a similar one within tolerance in a database of given size. Using the most commonly occurring letter pair "th" and characteristics specified by questioned document examiners, the highest probability formation and low probability formations in a database are determined. Computational issues in scaling the methods are discussed.