user: Question: What were the dates administered when the lead margin was larger than 14 and the poll source was Research 2000?
Columns: ['Poll Source', 'Dates administered', 'Democrat: Tom Harkin', 'Republican: Christopher Reed', 'Lead Margin']
Types: ['text', 'text', 'text', 'text', 'real']
Sample row: ['Survey USA', 'October 28-October 29, 2008', '61%', '35%', 26.0]
SQL:

assistant: SELECT "Dates administered" FROM "Table" WHERE "Lead Margin" > 14.0 AND "Poll Source" = 'Research 2000';