428 BELL SYSTEM TECHNICAL JOURNAL 



(2) 10 minutes have elapsed since the last attempt for the desired num- 



ber.— Code AB 



(3) Call is disposed of by being given to the operator. — Code PR 



(4) Call is disposed of by receiving a ''Don't answer" on an attempt to 



reach the desired number — Code DA 



AH attempts made during the period that an observation is ordinarily 

 held will be entered on the service observing detail sheets in the regular way. 

 In addition, these entries and entries showing any other attemps to reach the 

 desired number together with the proper code listed above to show the final 

 disposition of the call will be recorded on Form S.O. 171. 



In order to minimize the number of cases not completed at the end of an 

 observer's trick, no cases will be recorded on the special record on which the 

 original busy signal is received after J hour prior to the finish of any trick. 



From the instructions it may be noted that observations originating on 

 P.B.X. trunks or on coin lines were not included. The reason for this is, 

 when a busy is observed on a call originating on a P.B.X. trunk the subse- 

 quent attempt might be made on one of the other P.B.X. trunks, thus the 

 subsequent attempt would be missed. Also, at a P.B.X. two extensions 

 may place calls, within a few seconds of each other, to the same busy line. 

 The service observations on any one trunk might therefore be a mixture of 

 attempts involving two or more calls. When a busy is observed on a call 

 made from a coin line, the calling party will in many instances vacate the 

 coin box in favor of someone else, and the subsequent attempt may then 

 be made from another coin line. For these reasons the observations were 

 restricted to business and residential individual lines and to two-party lines 

 (12 observations were on two-party lines). 



It may also be noted that the observers were instructed to hold the line in 

 the observing position until ten minutes have elapsed since the last attempt 

 for the desired number. This was a departure from regular service observing 

 practices when a line is held until 1 minute has elapsed. 



Table II is a tabulation of the data observed at the Manhattan Service 

 Observing Bureau on Manhattan dial subscriber lines. The observations 

 are arranged in the order of increasing magnitude of the time inter\^als 

 between the start of the first attempt and the start of the second attempt. 

 Of interest is observation number 197 where a subscriber made 25 attempts 

 in about an hour. 



Data similar to that observed on Manhattan dial subscriber lines were 

 likewise observed on Bronx- Westchester and on Brooklyn-Queens dial 

 subscriber lines. 



Figure 1(a) shows graphically the data listed in Table II. This graph 

 shows, by dots, the cumulative percentage of the 451 Manhattan observa- 



