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Multiperson activity recognition in videos is a challenging task, due to the complexity of interactions among multiple persons. In this paper, a new statistical model, named coupled observation decomposed hidden Markov model (CODHMM), is presented to model multiperson activities in videos. A human activity that involves multiple persons is analyzed in two levels: the individual level that describes each individual's motion details and the interaction level that expresses the shared information among multiple persons. The two levels are modeled by two hidden Markov chains that are interdependent and interact with each other. The observation in each chain at each time slice is decomposed into subobservations according to the number of features and the number of persons. For each activity to be recognized, a CODHMM is built and model parameters are learnt by a generalized expectation maximization (EM) algorithm. Given an input video that contains an unknown activity, maximum likelihood algorithms are developed to classify it into one of the learnt activity categories. Experimental results show that the CODHMM can successfully classify human activities involving multiple persons with high accuracy and low computations.