user: ### NOW ANSWER:
Question: What is the Year Commissioned of the power stationo with a Gross head of less than 18?
Columns: ['Name', 'Year commissioned', 'Gross head (metres)', 'Installed capacity (megawatts)', 'Average annual output (million KWh)']
Types: ['text', 'real', 'real', 'real', 'real']
Sample row: ['Deanie', 1963.0, 113.0, 38.0, 92.0]
SQL:

assistant: SELECT AVG("Year commissioned") FROM "Table" WHERE "Gross head (metres)" < 18.0;