How many episodes were written by Alison McDonald?
CREATE TABLE "table1_26961951_4" (
  "no_in_series" real,
  "no_in_season" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "u_s_viewers_million" text
);
SELECT COUNT("directed_by") FROM "table1_26961951_4" WHERE "written_by"='Alison McDonald';