Returns a number that represents the standard deviation of a parents child records. However only the number of records specified by the Index parameter will be used. The OrderBy argument can be used to choose the order in which the records are selected.
SDEV_BYINDEX ( <field_name>, Index [, <orderby field>] [, <filter_query>] )
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field_name: The name of the field to use in the calculate of the Standard Deviation.
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index: The number of records to include in the calculation.
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orderby_field_name: The field on which to sort child records.
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filter_query: A query string representing the domain of records to include.
Example: Create a field on the CUSTOMER table that gives the standard deviation of the first 3 products sorted on purchase date.
SDEV_BYINDEX( [PRODUCT.PRODUCT_VALUE],3,[PRODUCT.PRODUCT_PURCHASE_DATE])