What were the dates administered when the lead margin was larger than 14 and the poll source was Research 2000?
CREATE TABLE "opinion_polling_for_the_united_states_se" (
  "poll_source" text,
  "dates_administered" text,
  "democrat_tom_harkin" text,
  "republican_christopher_reed" text,
  "lead_margin" real
);
SELECT "dates_administered" FROM "opinion_polling_for_the_united_states_se" WHERE "lead_margin">14 AND "poll_source"='research 2000';