Choose one item and list the sales transactions from Inventory History. Choose the same item and the same period in SalesAnalysis and list the sales. Then use GridView to inspect the OE history files. In OE all quantities are positive, and you must flip the sign based on the document type ("C" = Credit Note, "I" = Invoice). In SalesAnalysis the sign is flipped so just summing the quantities sold works. All modules allow people to purge data. SalesAnalysis allows people to edit the numbers (the data is considered statistical in nature).

Do you have alternate units of measure? SalesAnalysis normalizes the sales. OE and IC may not.