CREATE TABLE table_23447 ("Index  ( Year ) " TEXT, "Author / Editor / Source" TEXT, "Year of publication" TEXT, "Countries sampled" FLOAT, "World Ranking  ( 1 ) " TEXT, "Ranking L.A.  ( 2 ) " TEXT)

How many years were listed under index when there were 157 countries sampled?
SELECT COUNT("Index (Year)") FROM table_23447 WHERE "Countries sampled" = '157'
SELECT COUNT("Index (Year)") FROM "table_23447" WHERE "Countries sampled" = '157'