I am trying to use the sql reporting services and I cannot figure out how to use scope parameters

I created a report with various information about stores

If a grouping is specified, the running value is reset when the group expression changes.

I have a Group on =Fields!FECHA.Value in the dataset, and I want reset the field!Cost when Field!Fecha change

How change this expressions RunningValue(Fields!Cost.Value, Sum, Nothing) ?



Thank’s

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Yovanny Vasquez