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

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'