A leading scholar on the history of computer security, Donald MacKenzie, similarly points out that, "RAND had done some penetration studies (experiments in circumventing computer security controls) of early time-sharing systems on behalf of the government." Jeffrey R. Yost of the Charles Babbage Institute, in his own work on the history of computer security, also acknowledges that both the RAND Corporation and the SDC had "engaged in some of the first so-called 'penetration studies' to try to infiltrate time-sharing systems in order to test their vulnerability." In virtually all these early studies, tiger teams successfully broke into all targeted computer systems, as the country's time-sharing systems had poor defenses.