Among the crimes, what percentage are severe?
severe refers to index_code = 'I'; percentage = divide(count(iucr_no where index_code = 'I'), count(iucr_no)) * 100%
SELECT CAST(COUNT(CASE WHEN index_code = 'I' THEN iucr_no ELSE NULL END) AS REAL) * 100 / COUNT(iucr_no) FROM IUCR