user: Question: What is the average number of casualties for the convoy with a number of U-844?
Columns: ['Date', 'Number', 'Type', 'Captain', 'Casualties']
Types: ['text', 'text', 'text', 'text', 'real']
Sample row: ['16 Oct 1943', 'U-470', 'VIIC', 'O/L Grave', 46.0]
SQL:

assistant: SELECT AVG("Casualties") FROM "Table" WHERE "Number" = 'U-844';