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The transformation offers two options for calcuating the mean value over a period. One method (the default) takes the last n values that fit in the output interval (includding the time itself) and stores these at the output time. The alternate method (used by setting the includeFirstValueOfAggregationPeriodInCalculation option to true – this is the default behaviour) takes the mean of the last n pairs, averages that, and stores it at the output time.
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