The concept of episodes was introduced for discovering the useful and interesting temporal patterns from the sequential data. Over the years, many episode mining strategies have been suggested, which can be roughly classified into two classes: Apriori-based breadth-first algorithms and projection-based depth-first algorithms. As we know, both kinds of algorithms are level-wise pattern growth methods, so that they have higher computational overhead due to level-wise growth iteration. In addition, their mining time will increase with the increase of sequence length. In the paper, we propose a novel two-phase strategy to discover frequent closed episodes. That is, in phase Ⅰ, we present a level-wise shrinking mechanism, based on maximal duration episodes, to find the candidate frequent closed episodes from the episodes with the same 2-neighboring episode prefix, and in phase Ⅱ, we compare the candidates with different prefixes to discover the final frequent closed episodes. The advantage of the suggested mining strategy is it can reduce mining time due to narrowing episode mapping range when doing closure judgment. Experiments on simulated and real datasets demonstrate that the suggested strategy is effective and efficient.
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