Here's what I suggest:
Create two queries.
Query 1 is for your Sent Items:
SELECT Table1.CustID, DatePart("m",[DateSent]) AS MonthSent,
DatePart("yyyy",[DateSent]) AS YearSent, Count(Table1.CustID) AS
TotalShipped
FROM Table1
GROUP BY Table1.CustID, DatePart("m",[DateSent]),
DatePart("yyyy",[DateSent]);
Query 2 is for Returned Items:
SELECT Table2.CustID, DatePart("m",[DateReturned]) AS MonthReturned,
DatePart("yyyy",[DateReturned]) AS YearReturned, Count(Table2.CustID)
AS TotalReturned
FROM Table2
GROUP BY Table2.CustID, DatePart("m",[DateReturned]),
DatePart("yyyy",[DateReturned]);
Now join the two queries together in a third query using outer joins:
SELECT Query1.CustID, MonthName([Query1].[MonthSent]) & " " &
[YearSent] AS WhenSent, Query1.TotalShipped, Query2.TotalReturned
FROM Query1 LEFT JOIN Query2 ON (Query1.YearSent = Query2.YearReturned)
AND (Query1.MonthSent = Query2.MonthReturned) AND (Query1.CustID =
Query2.CustID);
You can also include the Customer name from your customer table if that
is what you want.
Use the third query as the record source for your table. Using the
report wizard will aid you in creating your report. You will want one
heading to be your customer Name, then When Sent. You will now need to
create a text box and in it put:
=Nz(NumberOfReturnedItems,0)/NumberOfSentItems.
Hope that helps!