user: Question: What is the average building year for Superb?
Columns: ['Name', 'Builder', 'Wheel arrangement', 'Works Number', 'Built']
Types: ['text', 'text', 'text', 'real', 'real']
Sample row: ['Alpha', 'W.G. Bagnall', '0-6-2T', 2472.0, 1932.0]
SQL:

assistant: SELECT AVG("Built") FROM "Table" WHERE "Name" = 'Superb';