The work load given to workers by a monotonous task was investigated. Their load was obtained by measuring the flicker value and observing its fluctuation pattern. Each flicker fluctuation was classified into one of the three patterns. It was found that the mental load of the subject carrying out the task by himself tends to become lighter according as the speed increases, while the mental load of the subject doing the task with his companion does not show this tendency. On the other hand, the scores of the disposition tests were analyzed with regard to the flicker fluctuation pattern by using the discriminant function

Kikuchi, Susumu

Ohgishi, Shinji

Osaki, Hirokazu

Okayama University Digital Information Repository

A Study of the Work Load in a Monotonous Task

Okayama University Scientific Achievement Repository

MEMOIRS OF THE SCHOOL OF ENGINEERING, OKAYAMA UNIVERSITY Vol. 5, No. I, SEPTEMBER 1970A Study of the Work Load in a Monotonous TaskSusumu KIKUCHI, Hirokazu OSAKI and Shinji OHGISHIDepartment of Industrial Science(Received May 20, 1970)SynopsisThe work load given to workers by a monotonous task was investigated.Their load was obtained by measuring the flicker value and observing itsfluctuation pattern. Each flicker fluctuation was classiried into one of thethree patterns. It was found that the mental load of the subject carryingout the task by himself tends to become lighter according as the speed in-creases, while the mental load of the subject doing the task with his companiondoes not show this tendency. On the other hand, the scores of the disposi-tion tests were analyzed with regard to the flicker fluctuation pattern by usingthe discriminant function.§ 1. IntroductionIn the modern world, where various kindsof capital and machinery have been playing aleading role, the conveyor system has beenconsidered to be most efficient when its speedis settled so as to economize the line control.In the very present time, however, the gapbetween the efficiency of machines and thehuman adaptability to environments has be-come great. Consequently, it will be best todecide the conveyor speed not only by takinginto account the economization of the linecontrol, but also, from the viewpoint of laborcontrol, by trying to keep the worker's bodyand mind as sound as possible.The present experiment was carried out inorder to find the most effective conveyor speedin a very monotonous kind of work. The idealspeed was decided by examining the fluctua-tions of flicker value that the subject showed asthe result of the various given speeds of hismonotonous task. The fluctuation of flickervalue is generally considered to be indicativeof the worker's mental load in the monotonoustask, namely, the task which does not requireso much physical and mental labor.The experiment proceeded as follows. Thesubject was made to do the monotonous taskunder three different working conditions. Theflicker value of the subject under each \vork-ing condition was measured in order to knowhow much the subject was influenced by the27task. Next, the difference of the influence,that is, work load given to the subject, wasexamined between when the subject did thetask by himself and when he did the same taskwith another subject sitting next to him. Inorder to estimate the work load of the subject,the interrelation between his scores of disposi-tion tests and the fluctuations of his flickervalue was investigated.§ 2. MethodsThe monotonous task chosen for this experi-ment consisted of placing cards at indicatedpositions. Each card was 30mm square in sizeand had one of the numeral letters I to 5 writ-ten on it. Every card was kept in the box tillit was taken out and placed on the square ofthe same size drawn at even intervals on asquare board (500 X 500). This drawn squarealso had one of the I to 5 numerals written onit (Fig. 1). The subject was required to placeeach card on the drawn square with the samenumeral letter that the card had. He was alsorequired to arrange the cards in order begin-ning from the top left of the board. Theperiod between his taking up the card and thenext same act was settled even (work speed) ;he was informed of each time to place the cardat the proper position by an intermittent puresound (500 Hz/sec for 0.5 sec.). The momenthe heard the sound of signal, the subject placedthe card at the proper position, and then hewas permitted to seek out the next proper card28 S. KIKUCHI, H. OSAKI and S. OHGISHISUbjectFig. 1 Arrangement of the subject andexperimental apparatustuation as time series. 0,2)PI: The flicker value always became higherthan the intial one, that is, the flickervalue before the task. This signifiesthat the mental load to the subjects wassmall.P 2: The flicker value became higher orlower than the initial value. This im-plies that the mental load was moderate.P 3: The flicker value became lower andlower after the beginning of the task.This shows that the mental load wasconsiderably large.Fig. 2 shows each typical pattern of thesethree kinds.Fig. 2 Typical patterns of flicker fluctuation-4.0'O~--L""'1~O--'--;2:!;.O"'"""-;3:!;.O"'"""-47;O~-;5""O~-6r!;O"--­Time (min,)P3PIi\ PP2f ' ,o / b" Ib- .... ,,' '0... /\ / '-,1\ fl!3.0-3.0-20-1.0..<1>x.~\Lu<1>.!!! 20<1>U>.uTable 1 Distribution of the subjects in flickerpatterns. Tbe numbers in parentheses showvalues revised so as to make each total num-ber 32.:~ I Total numberWoek P 1 P 2 P 3 of subjectsspeedC 1 0 8 19 27( 0) ( 9) (23) (32)--C 2 10 12 10 32-IC 3 13 13 4 30(14) (14) (4) (32)(I) Working by oneselfThe results of the task done separately byeach subject under three different workingconditions were classified as shown in Table 1.The results in the case C I (slow pace) mostly§ 3. ResultsThe fluctuations of flicker value during thetask were classified into the following threepatterns (P 1, P2, P3) in order to know thedegree of the work load by grasping the fluc-and wait for the following signal.The three different working conditions werethat the subject should arrange 50 cards in 5min. (CI), 90 cards (C 2) and 110 cards (C3) inthe same period. In the case of CI, the sub-ject had to wait for the following signal for aconsiderable time. The work speed in C 2 wasmoderate enough for the subject to do his taskwith ordinary efforts. In the case of C 3, how-ever, the subject had to make great effortsin order to arrange cards exactly according tothe signal.The total task period of one subject was 60minutes, and each period consisted of 12 ses-sions of continuous 5 minutes each. The flickervalue was measured five times after each ses-sion, and its mean value was calculated.The subjects were male students of 20-23of age. Each one was made to do the giventask under one working condition in a day sothat the result might be as exact as possible.The subject was kept by himself during histask, and he was observed with a video camerafrom the next room, where measurementswere carried out.In order to estimate the work load of eachsubject by using the discriminant function,Taken disposition test and Yatabe-GuilJord (Y-G) test were made on every subject, becausedisposition is considered to have something todo with the work load.A Study of the Work Load in a Monotonous Task 29Table 3 Error probability of pattern classiEca-tion (Taken)%34C 331C 2l' 2 - l' 3~~\\o~~ speed I C ICroup ~'c----,-----.--;--.....,-P I - l' 2~~ fatternP 1 P 2 I P 3 Total numberWork~tof subjects_speed8I j 40C I I 18 141-- 1---C 3 12 , 13 I 13 38(12) ! (14) 1 (14) (40)27 I 40-------1------1--- 1----1'3-1'1 I 38Table 2 Distribution of the subjects in Rickerpatterns. The numbers in parentheses showvalues revised so as to make each total num-ber 40.the working condition, almost the same num-ber of subjects showed each one pattern of thethree, namely, P I, P 2, or P 3. This impliesthe subject's mental load \I'as affected by thepresence of his companion, especially underthe s'OW working condition (C I). This will beeasily understood, because it is quite naturalthat the subject should feel less tired of hismonotonous task while working with his com-panion than when he does it by himself.It may be concluded that the work loadgi\'en to the sub~ect by the task has a consider-ab~e re'ation \\lith the environments in whichhe does his task and that the work load is in-dividually different.(3) Estimation of the flicker fluctuation pat-tern by means of the discriminant functionIn the first part of this section, the subjects'flicker fluctuations were classified into threepatterns, namely, PI, P2, and P3. However,with the purpose of estimating the flicker fluc-tuation pattern without carrying out theabove-mentioned experiment, the followinganalysis was attempted. The scores of disposi-tion tests obtained by each subject were ana-lyzed by using the linear discriminant func-tion, and all the subjects were discriminatedand divided into each two pair groups, name-ly, PI and P2, P2 and P3, and P3 and PI.The error probability (a) of this discrimina-tion is shown in Table 3 (Taken) and Table 4(V.G).come under P 3 (falling pattern) and not at allunder P I (rising pattern). This may seemcontradictory, but is considered to be quitenatural. It is true that the physical load wasleast under C 1, but the subject was compelledto wait for a considerable while between theacts of arranging cards. This waiting is con-sidered to have made the subject feel muchthe monotony of his task, and as the result hismental load became great. Consequently, hisflicker value went down.In the case of C 2 (moderate pace), the reosults come under three patterns almost equally.This shows that the subjects' mental load wasdifferent from one another's, although theamount of each one's physical labor was thesame. One of the reasons for such a result maybe that all the subjects were students who hadnever done such a monotonous task of arrang-ing cards at a given interval and were apt torespond variously to their working environ-ments. Therefore, this difference may havebeen caused by the difference in disposition ofthe subjects.The results in the case of C 3 (fast pace) arecontrary to those in the case of C 1, that is,there are few that come under P 3 (falling),and many come under P 1 (rising). This showsthat the subjects' flicker value was high, inother words, they did not feel so much themonotony of their task in spite of the greatamount of their physical labor and the greatestefforts they had made.From these results it was found that themental load tends to be lighter accordingas the working speed increases. It may beconcluded that the mental load of the workerhas a close relation with the speed at whichhe carries out his monotonous task.(2) Doing the task with another subjectWhen two subjects did the same task of ar-ranging cards, sitting side by side and talkingfreely with each other, the results were asfollows. These are shown in Table 2.In the case where they worked with a slowpace (C I), there were more subjects whoshowed the rising pattern (PI) than when theyworked separately. In the case of C 3, on thecontrary, the number of the subjects showingthe falling pattern (P 3) increased, comparedwith that in the case of working by oneself. Inother words, regardless of the difference in30 S. KIKUCHI, H. OSAKI and S. OHGISHITable 4 Error probability of pattern classifica-tion (Y-G)~~~ speed I C I C 2 C 3Group ~I%P I - P 2 - 11 15~-~P 2 - P 3 20 12 -P 3 - P I - I 24 -The discriminant function of each of thetwo tests, Taken and Yatabe-Guilford, is as fol.lows:Y=A1X 1+A2X2+··· +A5X5(Xt : Scores of Taken disposition test consist-ing of five items)Z=B1Y 1+B2Y2+··· +B12 Y 12(Yt : Scores of Y.G disposition test consist-ing of twelve items)For example, the discriminant function be-tween P I group and P 2 group in the case ofC 2 is as follows:Z=0.019Yt-0.414Y2-0.101Y3+0.363Y4-0.168)'5+0. 269Y6+0. 255Y7-0. 481Y8-0 .108Y9+0. 374ylO-0. 001;>11-0 .477)'12The mean of Z of P I group is -4.17 and thatof P 2 is -10.24, and the discriminant pointis -7.21. When the value of Z, which wasdetermined by substituting Yi with scores ofthe disposition test, is larger than the discrim-inant point, the subject's pattern is judgedto be P I. When Z is smaller than the discrim-inant point, the pattern is considered to beP 2. And the error probability in this case is11%.The error probability in such a pattern clas-sification according to the scores of the Y-Gtest is smaller than 25 %. As there were nopatterns of P 1 type in the case of C I and fewin the case of C 2, the discriminant functionin these cases were not calculated.When the Taken test was employed, eacherror probability was larger than 25 %, andconsequently this kind of test is consideredto be inferior to the Y.G test.§ 4. ConclusionThe degree of the work load given to subjectsby a monotonous task was studied by measur-ing their flicker values. Furthermore, theirscores of disposition tests were analyzed byusing discriminant functions with regard totheir flicker fluctuation patterns.The conclusions obtained from these resultsare as follows:I. The degree of the mental load of a workercarrying out a monotonous task depends uponthe working speed or upon environmentalconditions: When the worker does the task byhimself, his mental load tends to become lighteraccording as the working speed increases, butwhen he does the task with his companion,the mental load tends to be light especiallyat a slow working speed.2. It is possible to classify the workers'flicker fluctuation patterns by analyzing theirscores of the Y-G disposition test with thelinear discriminant function.ReferencesI) H. OSAKI and S. KIKUCHI: Memoirs School Eng.Okayama Univ. 2, (1967) 262) H. OSAKI and S. KIKUCHI: Memoirs School Eng.Ckayama Univ. 4, (1969) 313) E. GRANDJEAN: Fitting the Task to the Man, Taylor& Francis Ltd, (1969)

A Study of the Work Load in a Monotonous Task

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