In 1981, the Physicians Health Study sent participation invitation letters to all 261,248 male physicians between 40 and 84 years of age who lived in the United States and who were registered with the American Medical Association.

Of the male physicians invited, less than half responded and slightly over 59,000 were willing to participate. Of the 59,000 male physicians willing to participate, 33,223 were eligible and 11,152 changed their minds to participate. The final study number included 22,071 physicians who were then randomized to experimental groups and followed for the duration of the study. The study was restricted to physicians in order to facilitate follow-up, since all subjects were registered physicians in the AMA. The study excluded female physicians, because in 1981 the number of registered female physicians over the age of 40 was quite small and would not have provided enough statistical power to provide valid results in females.

Please describe how external validity of this experimental study may have been affected if results were generalized to all physicians in 1981.