How many years were listed under index when there were 157 countries sampled?
CREATE TABLE "other" (
  "index_year" text,
  "author_editor_source" text,
  "year_of_publication" text,
  "countries_sampled" real,
  "world_ranking_1" text,
  "ranking_l_a_2" text
);
SELECT COUNT("index_year") FROM "other" WHERE "countries_sampled"=157;