This model is reliable in mathematical principle and it is easy to use the formulae.

该模型数学原理可靠，公式运用简便。

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First and foremost, the two indices use different averaging formulae when calculating price changes for each individual item.

第一，这两个指数在计算单项产品的价格变化时套用了不同的平均值公式。

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The pricing formulae for copper, which allow buyers to set prices after the cargo arrives, as well as the extensive use of hedging instruments, have helped blunt some of the price gyrations.

铜的定价模式和对冲工具的广泛运用缓和了价格波动，这种模式允许买家收货后再定价。

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First and foremost, the two indices use different averaging formulae when calculating price changes for each individual item.

ECONOMIST: The switch in price indices

The main difference is caused by the fact that, although they use much of the same data, they calculate the inflation rate using different formulae.

BBC: Q&A: Inflation explained