CREATE TABLE table_15481 ("Institution" TEXT, "Location" TEXT, "Founded" FLOAT, "Type" TEXT, "Joined" FLOAT, "Nickname" TEXT, "Colors" TEXT)

When did Northeastern University join?
SELECT SUM("Joined") FROM table_15481 WHERE "Institution" = 'northeastern university'
SELECT SUM("Joined") FROM "table_15481" WHERE "Institution" = 'northeastern university'