Name the average points for chassis of stevens and year less than 1950
CREATE TABLE "complete_formula_one_world_championship_" (
  "year" real,
  "entrant" text,
  "chassis" text,
  "engine" text,
  "points" real
);
SELECT AVG("points") FROM "complete_formula_one_world_championship_" WHERE "chassis"='stevens' AND "year"<1950;