Please answer a question about the following article about IBM:

In 1952, Thomas Watson, Sr., stepped down after almost 40 years at the company helm; his son, Thomas Watson, Jr., was named president. In 1956, the company demonstrated the first practical example of artificial intelligence when Arthur L. Samuel of IBM's Poughkeepsie, New York, laboratory programmed an IBM 704 not merely to play checkers but "learn" from its own experience. In 1957, the FORTRAN (FORmula TRANslation) scientific programming language was developed. In 1961, Thomas J. Watson, Jr., was elected chairman of the board and Albert L. Williams became company president. The same year IBM developed the SABRE (Semi-Automatic Business-Related Environment) reservation system for American Airlines and introduced the highly successful Selectric typewriter.

Thomas Watson Sr. stepped down in what year?
1952