CREATE TABLE table_788 ("District" TEXT, "Incumbent" TEXT, "Party" TEXT, "First elected" FLOAT, "Result" TEXT, "Candidates" TEXT)

What year was incumbent william f. stevenson first elected?
SELECT "First elected" FROM table_788 WHERE "Incumbent" = 'William F. Stevenson'
SELECT "First elected" FROM "table_788" WHERE "Incumbent" = 'William F. Stevenson'