How many percent of senators were from class 1?
senator refers to type = 'sen'; class 1 refers to class = 1; calculation = MULTIPLY(DIVIDE(COUNT(class = 1 then bioguide), COUNT(bioguide)), 1.0)
SELECT CAST(SUM(CASE WHEN class = 1 THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(*) FROM `historical-terms` WHERE type = 'sen'