356 BELL SYSTEM TECHNICAL JOURNAL 



which may not have been shown by electrical test, and provide a close 

 check on the testing personnel. Quick access to the final index per- 

 mits intelligent control during the testing program. Some of the tests 

 planned may be dropped and others added according to the nature of 

 the data. 



Fig. 2 — Interior of observing booth. 



Another important benefit is the control provided over the inev- 

 itable changes in the skill of the testing crew. Even the most experi- 

 enced crew is likely to show a quick growth in proficiency in the early 

 stages of testing a strange circuit, or may display a temporary loss of 

 skill when returned after some time to a circuit previously familiar. 

 During these stages the data ought usually to be discarded. The 



