user: Question: How many years have HMOs been 27% and POS plans 18%?
Columns: ['Year', 'Conventional plans', 'HMOs', 'PPOs', 'POS plans']
Types: ['real', 'text', 'text', 'text', 'text']
Sample row: [1998.0, '14%', '27%', '35%', '24%']
SQL:

assistant: SELECT COUNT("Year") FROM "Table" WHERE "HMOs" = '27%' AND "POS plans" = '18%';