The decision to utilize only military test pilots proved to be a sound one.  According to Dr. Gamble:
 

They were a good, solid group, largely preselected, and preexperienced.  And also, their records were available in Washington for preliminary screening, which was not true of Industry, or civilian test pilots.  Furthermore, they had, most of them, graduated from the military test pilot schools where there are high standards for entrance and even higher requirements for graduation.  And their Job is very nearly similar to that of an astronaut during the first flights. Furthermore, they were familiar with the full-pressure suits and complex cockpits.  And we had one thought that perhaps many of you might not have considered; they would then be a homogeneous team, including their wives.⁹

By January 1959 the selection committee was ready to review records of possible candidates, including all test pilots on active duty.  This work was done by the "Phase I" group, whose names are indicated in the following paragraphs.  Full cooperation was given by military officials in this work, which required several weeks.  More than 500 names were selected for further consideration, including over 200 Air Force test pilots and 200 Navy pilots, 23 Marine pilots, and 40 Army pilots.  NASA announced on January 28, 1959, that 110 had met all the basic requirements."¹⁰

On Monday, February 2, 1959, 69 reported to Washington under special military orders. On that day and the following Monday, they attended briefings that included a detailed technical explanation of the problems involved.  Later in the day they came back for individual interviews during which they were asked to volunteer or decline.  No record was kept on those who declined.¹¹

There were several types of measurement for those who volunteered.  First was a joint technical interview by Charles J. Donlan, Assistant Director of Project Mercury (an engineer) ; Warren North of the Space Flight Program (a former test, pilot.); and Dr. Gamble, the Manpower Evaluation Development Officer (an industrial psychologist).  During these sessions further technical details of Project Mercury were made available to the candidate, including engineering drawings and specifications, and the individual pilots were encouraged to inquire in depth in areas of their interest.. This yielded valuable clues concerning motivations and technical backgrounds.

The second test was a psychiatric evaluation by two psychiatrists  who were Air Force officers, Dr. George E. Ruff and Dr. Edwin Z. Levy.  Each recorded his independent conclusions; they compared notes, and then they reported to the committee.  The third test was a detailed review of the medical records and a medical interview by the flight surgeon, Dr. Augerson. In addition, Dr. Voas gave the candidates a battery of written tests including the Miller Analogies Test (a graduate-school-level test), the Minnesota Engineering Analogies Test, and the Doppelt Mathematical Reasoning Test. Others who helped conduct Phase I were Dr. White, Dr. William F. O'Connor, and Dr. David K. Trites, a Navy officer.

Thirty-two pilots were chosen for Phase II, which was to be carried out at the Lovelace Clinic, Albuquerque, N. Mex. Factors in the choice of this facility were the work it was doing for the USAF Air Research and Development Command (later Systems Command) on selection techniques for the Man-in-Space program and the fact that it had recently completed development of machine cards for recording all medical and related information from the Lovelace Foundation and Clinic and from the Aero-Medical Laboratory stress and related tests at Wright-Patterson AFB.

10.  Ibid. Also personal discussions with Drs. Voas and Gamble by the author.

11.   According to Dr. Gamble, advance estimates by the Committee had been that the rate of volunteers would range from 5 to 50 percent.  Actually it was more than 75 percent ("The Astronauts and Project Mercury," op. cit.).