SELECT avg(Order_Quantity) FROM Invoices WHERE payment_method_code  =  "MasterCard"
SELECT avg(Order_Quantity) FROM Invoices WHERE payment_method_code  =  "MasterCard" <table_name> : Invoices col : Invoice_ID | Order_ID | payment_method_code | Product_ID | Order_Quantity | Other_Item_Details | Order_Item_ID row 1 : 128 | 14 | MasterCard | 4 | 2 |  | 1 row 2 : 162 | 13 | MasterCard | 9 | 2 |  | 9 row 3 : 164 | 7 | Visa | 7 | 2 |  | 1 row 4 : 212 | 8 | Visa | 4 | 2 |  | 8 row 5 : 272 | 10 | American E | 3 | 2 |  | 5 row 6 : 398 | 3 | American E | 4 | 1 |  | 5 row 7 : 406 | 14 | MasterCard | 7 | 2 |  | 1 row 8 : 483 | 7 | Visa | 8 | 1 |  | 3 row 9 : 548 | 4 | MasterCard | 3 | 1 |  | 1 row 10 : 587 | 1 | American E | 4 | 2 |  | 1 row 11 : 612 | 6 | American E | 3 | 1 |  | 8 row 12 : 633 | 11 | Visa | 2 | 1 |  | 7 row 13 : 679 | 12 | MasterCard | 9 | 2 |  | 8 row 14 : 733 | 11 | Visa | 6 | 1 |  | 4 row 15 : 777 | 7 | MasterCard | 6 | 2 |  | 7
col : avg(Order_Quantity) row 1 : 1.8333333333000001