The Previous function returns the previous value for the expression evaluated in the specified scope after all sorting and filtering have been applied.
If expression does not contain an aggregate, the Previous function defaults to the current scope for the report item. For a text box in the details group, =Previous(Fields!Quantity.Value) returns the data for the field Quantity from the previous row. This expression in the first row returns a null (Nothing in Visual Basic).
If expression contains an aggregate function that uses a default scope, Previous aggregates the data within the previous instance of the scope specified in the aggregate function call.
If expression contains an aggregate function that specifies a scope other than the default, the scope parameter for the Previous function must be a containing scope for the scope specified in the aggregate function call.
The functions Level, InScope, Aggregate and Previous cannot be used in the expression parameter. Specifying the recursive parameter for any aggregate function is not supported.