When did Northeastern University join?
CREATE TABLE table_15481 (
    "Institution" text,
    "Location" text,
    "Founded" real,
    "Type" text,
    "Joined" real,
    "Nickname" text,
    "Colors" text
)
SELECT SUM("Joined") FROM table_15481 WHERE "Institution" = 'northeastern university'
Below are sql tables schemas paired with instruction that describes a task. Using valid SQLite, write a response that appropriately completes the request for the provided tables. ### Instruction: When did Northeastern University join? ### Input: CREATE TABLE table_15481 (
    "Institution" text,
    "Location" text,
    "Founded" real,
    "Type" text,
    "Joined" real,
    "Nickname" text,
    "Colors" text
) ### Response: SELECT SUM("Joined") FROM table_15481 WHERE "Institution" = 'northeastern university'