Rather than conditionally selecting records based on the store ID, I'd use that parameter to surpess stores which don't equal that store ID.
IE You need all stores to be in your data to analyzze them for the national average.
Create a group on StoreID, format this group how you'd like your individual store to be displayed. Now conditionally surpress the group on
database.storeid <> {?storeid}
Now in your report footer you can create national averages as all the data for other stores is still in the report.
Regards,
Ryan.