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This transformations calculates the mean value of instantaneous values over a certain period. If the option allowMissingValues is true (this is the default behaviour), then a missing value is returned if one of the input values in the period is a missing value. If the option allowMissingValues is false (this is the default behaviour), then a mean value is calculated if there are 1 or more non-missing values in the aggregation period, i.e. missing values are ignored in this case.
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