What are the average points of Scuderia Scribante with a Brabham bt11 chassis before 1968?
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 "entrant"='scuderia scribante' AND "chassis"='brabham bt11' AND "year"<1968;