268 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



conditions to be expected. Insofar as practicable, measurements and obser- 

 vations of such factors as volume levels, dialing accuracy and speed, an- 

 swering times, and the number of '^don't answers" were made. 



Two separate trials were made of the 500-set. The first in the summer and 

 fall of 1948 was on a relatively small scale with some 50 pre-production 

 models. These were used intensively, at first to sample public opinion on 

 appearance and performance factors, and in the later phases to provide 

 strictly comparable service experience by some 100 selected subscribers, 

 half of whom used new present standard sets and half the 500-set. 

 After some weeks the sets were interchanged between the two groups, so 

 that each of the trial group of subscribers used each type of set. This trial was 

 intended to disclose quickly any significant factors requiring immediate 

 changes in manufacturing planning for production. The results of the trial 

 were so favorable to the new design that it was decided to proceed, using 

 early production sets for subsequent trials. The initial trial estabhshed an 

 overall preference for the new set by some 90% of the users. It also indi- 

 cated that every one of the new features was noticed and favorably com- 

 mented on by a substantial majority of the subscribers. All told, some three 

 hundred persons in four operating company areas saw and used these fifty 

 sets, and expressed preferences. 



In addition, performance data obtained during the trial confirmed en- 

 gineering expectations. Such factors, of course, as determination of the 

 relative maintenance effort required had to be left for a larger scale trial over 

 a much longer period. 



This more comprehensive trial was started with the first four thousand 

 production sets in November 1949. Ten locations in the territories of six 

 Bell System Associated Companies were chosen. In this choice, range of 

 climatic conditions was represented, from Manhattan and Staten Island on 

 the east coast, to San Francisco and Los Angeles on the west, and 

 from Chicago and St. Paul in the north to New Orleans in the south. In 

 these various places, step-by-step, panel, crossbar and P.B.X. connected sta- 

 tions, business and residence, individual and party, measured and flat rate, as 

 well as multi-party rural conditions, were included. Many sets were installed 

 on normal inward movement, some on cutover from manual to dial, and some 

 by substitution. In fact there was a deliberate and successful attempt to 

 include in the trial a broad coverage of all important conditions experienced 

 in plant and commercial operation. 



For each 500-type set installed a newly made set of the present standard 

 302-type was installed in a comparable location and by the same plant 

 forces. In this way, a balanced exposure to conditions for both types was 

 insured. 



Samples of each type of set were examined and measured in the laboratory 



