Context: William A. Thorpe was killed in an accident in 1918, his business continuing to operate on the island to the present day. In 1920 the Norwegian ship Spangereid caught fire and sank at her mooring at James Bay, depositing quantities of coal on the beach below the wharf. A census in 1921 showed the islands population was 3,747. The first islanders left to work at Ascension Island in 1921, which was made a dependency of St Helena in 1922. Thomas R. Bruce (postmaster 1898–1928) was the first islander to design a postage stamp, the 1922–1937 George V ship-design—this significantly contributed to island revenues for several years. South African coinage became legal tender in 1923, reflecting the high level of trade with that country. There were nine deaths from whooping cough between 1920 and 1929 and 2,200 cases of measles in 1932. The first car, an Austin 7, was imported into the island in 1929. A census in 1931 showed a population of 3,995 (and a goat population of nearly 1,500). Cable & Wireless plc absorbed the Eastern Telegraph Company in 1934. Tristan da Cunha was made a dependency of St Helena in 1938.

Question: How many years after Ascension Island was made a dependency of St Helena was Tristan da Cunha made a dependency?

Answer:
16