利用在更换周期点上具有的马尔可夫性质，以系统首次故障前平均工作时间最大为目标建立了确定最佳更换周期的模型。

Using the property of Markov at the points of replacement period, a model of determining the optimal period is developed with objective of maximizing the mean time to first failure of system.

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整体而言，周平均工作时间比起衰退前大幅下滑。

On the whole, average weekly hours are far below their pre-recession level.

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