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LABORATORY STUDIES OF REPETITIVE WORK. II :PROGRESS REPORT ON RESULTS FROM TWO SUBJECTSK. F. H. MURRELL a & BEL FORSAITH aa Department of Psychology , Unit for Research on Human Performance in Industry, Universityof Bristol. (Now at Welsh College of Advanced Technology, Cardiff.)Published online: 27 Apr 2007.

To cite this article: K. F. H. MURRELL & BEL FORSAITH (1963) LABORATORY STUDIES OF REPETITIVE WORK. II :PROGRESS REPORT ON RESULTS FROM TWO SUBJECTS, International Journal of Production Research, 2:4, 247-264, DOI:10.1080/00207546308947829

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247

LABORATORY STUDIES OF REPETITIVE WORK.

II PROGRESS REPORT ON RESULTS FROM TWO SUBJECTS

K. F. H. MURRELL* AND BEL FORSAITH*

SUMMARY

This experiment is the first of a series undertaken to test proposals put forward earlier by thefirst author that in repetitive work (i) output would be higher and variability less if breaks weregiven at the end of an actile period (period of optimum performance) rather than at the pointwhen output decreased; (ii) the end of an actile period would be indicated by the onset of irreg­ularity in performance which could be determined by the incidence oflongcycle times in unpacedwork and an increase in the number of missed cycles in paced work; and (iii) the length of anactile period will depend on the demand made by the job and upon the capacity of theworker to meet the demand (her actility),

Results from two subjects tested for a period of 31 months suggest that proposal (i) iscorrect. A condition in which two breaks were given at times indicated by an increase in longtimes in unpaced performance gave a significantly better output and lower variance than acondition in which one break was given at a time indicated by a fall in output. Both subjectshad relatively fewer misses in the paced condition when three breaks were given than whenworking continuously.

The technique for obtaining long times in unpaced work which formed part of (ii) wasfound to be practical and a simpler method was devised. The results obtained seem to confirmthat the method has validity. No definite conclusions can be drawn on paced work

There is some tentative confirmation of (iii); in the paced condition the actile period seemsto have been I hour compared with IJ hours in the unpaced condition. Further, the betterworker gave her best unpaced performance when rest was given after 1J hours whereas theless good worker did best when rest was given after I hour.

INTRODUCTION

Following the investigations of the Industrial Fatigue/Health Research Board betweenthe wars, the value of breaking up periods of work has been largely accepted in industry.As a rule, however, the number and location of the breaks are determined on administrativegrounds, while the break itself is almost invariably a rest. This is not unreasonable in viewof the findings of the Industrial Fatigue/Health Research Board investigators in a number ofad hoc studies that there were increases in output when rest was given; they also made thesuggestion that this rest should be given when output began to fall (Wyatt, 1927), but theyhad not produced any theory of performance which would act as a guide to deciding ifand when a break was required in continuous work.

After a close study of the literature Murrell (1962) suggested that in all work of anunvarying character there was a period of optimum performance (which he proposed tocall the octile period) at the end of which performance would start to disintegrate. Thisdeterioration would not necessarily have a corresponding effect upon the average rate ofwork. He further suggested that the optimum point in time for the introduction of a breakwould be at the end of the actile period and that, after a break, a fresh actile period wouldstart.

• Unit for Research on Human Performance in Industry, Department of Psychology, University ofBristol. (Now at Welsh College of Advanced Technology, Cardiff.)

Received by Int. Jnl. Prod. Res., August 28, 1963.

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248 REPETITIVE WORK: REPORTS ON TWO SUBJECTS

A series of experiments, of which the one now reported is the first, has been started inorder to test the operational description of the actile period as it relates to repetitive work.It is predicted that in an unpaced task the end of the actile period could be identified bythe onset of irregularities in the performance (to be determined by a method which wasoutlined) and/or in appropriate circumstances by an increase in errors (or decrease inquality); in paced work there would be a corresponding increase in the number of "misses".Breaks introduced at the end of the actile period should result in faster and more regularwork in an unpaced condition and a reduction in the number of misses in a paced condition.

Each experiment uses two subjects and takes 3--4 months to complete. The amount ofdata generated is substantial (approximately a quarter of a million cycles) so that it isappropriate to report on the progress of the series at convenient stages. Apparatus troublesduring the present experiment made re-design necessary, so that the results from the firsttwo subjects will not be strictly comparable with those from the remainder of the series.They are, therefore, reported separately here.

DESCRIPTION OF THE EXPERIMENT

Apparatus

The experimental task was intended to simulate the industrial job of repetitive inspectionof electrical components in which the component is placed in a fixture and a reading is taken.This type of task was chosen because it is simple and easily learned, because it containedperceptual clements which might give an error as well as a speed score and because it givesa convenient cycle time of between 7-9 sees,

The apparatus is illustrated in Fig. I. At A were six holes, closed by a flap which could beretracted by a solenoid operated by a P.O. key (D). At F were ten lights only one of which

Fig. 1. The apparatus.

was illuminated at a time. The lights "revolved" clockwise round the circle to simulate therevolving of a machine the speed of which could be varied at will by the experimenter.

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K. F. H. MURRELL AND BEL FORSAITH, 249

In the self-paced condition this was always faster than S could complete her task and the"revolving" would stop at light G and be restarted with key B. In the paced condition thelights "revolved" continuously but light G remained on for 2(11ths of the cycle time.Electrical interlocks isolated keys C and D unless key B was operated while light G was on.Thus in the paced condition the subject had to complete her task within the 2/1lths of thecycle time that the light G was on and if she failed to do this she could not proceed with thetask but had to wait until the light had reached G again.

E was a Jaquet electromagnetically operated stop-clock with a scale grad uated to 10m.secs.which was run by key C and reset by key D. The clock hand made one revolution in 3 sees.It was originally intended to run two of these clocks in parallel so that the experimentercould check the accuracy with which the S operated the clock, this idea had to be abandonedwhen it was found that there was variable error between the two clocks.

The S's task was to operate key B and then key C to make the clock run, stopping it asexactly as possible at 3 sees. Six screws were then picked up, one at a time, from the tray(H) and placed in the six holes (A). Key D was then operated which reset the clock andreleased the screws back into the tray. The sequence was then repeated. Ss worked one­handed with the preferred hand.

Recording

A special recorder was built for the experiment which could give the frequency distributionof the cycle times generated in any chosen interval of time (in this experiment in each15 mins.). In addition, a three channel pen-recorder was used. This pen recorder alonewas used in the paced condition.

The subjects

Ss were two married women in their early thirties, both of whom had worked in industrybefore marriage. Subject A attended from 09.00 hrs, to 13.00 hrs. and Subject B from13.00 hrs, to 17.00 hrs. for five days a week. They were paid a weekly wage with no bonus.

The experimental plan

Ss started work after Easter and continued until early August 1962. 3t hours work wascarried out during each 4 hour period of allendance. The original programme was disruptedby component failure, any failure causing the slightest interruption during a 3t hour runmeant that the results of that run were useless. As a result of failure from one cause oranother A lost 31 days and B 22 days out of 77 days testing so that insufficient recordswere obtained under the later conditions.

Recording started after two weeks of practice during which the Ss were allowed to restwhen they wanted to. The following conditions were then tested:

UP.c.P.c.UP.xB.yP.xB.y

Unpaced, continuous (with no breaks)Paced. continuousUnpaced, with x breaks the first at y hours after the start.Paced, " "

UP.c. was tested first, 8 complete records were obtained for subject A and 10 for subjectB. While calculations were being made which would enable the UP.c. conditions to betested the P.C. condition was run, starting at an output rate which approximated to theaverage output in the UP.c. condition. Subject A did 5 successful days at hourly rates from345-383 and subject B did 10 days at hourly rates from 439-487. These no break days were

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250 REPETITIVE WORK: REPORTS ON TWO SUBJECTS

then followed by a series of days (self-paced and paced) with rest inserted at various times,and towards the end several UP.c. conditions were repeated to see whether there had beenany change in the output rate.

RESULTS: CONTINUOUS CONDITIONS

Unpaced (UP.c.)

After two weeks practice both Ss went straight on to 31 hours continuous work. Atfirst thcy seemed to find this period of work somewhat of a strain but after about 7 dayswork (interrupted on some days by breakdowns) they achieved a high output (Fig. 2).

SUBJECT A\\\

\J", .... ,\ ,I •

11001--- .....

1200

1300

18 19 24 25 26 27APRIL

3 7 9 10MAY

SUBJECT B

\\

-'--19 20JULY

Fig. 2. Output in successive 3 ~ hour continuous shifts.

Thereafter the output declined somewhat, followed in the ease of A by a further increase.The series ended on May II th and on re-test two weeks and then two months later theouptut was found to be substantially lower than the level previously achieved. Sincequantitative results were not a primary objective of the experiment the exact level of per­formance is not of vital importance. The output and hourly rates are given in Table I.

Table I. OU/filII and hourly rate under UP.c.

Subject Date Mean output Mean hourly No. oroutput shirts

A 16/4 to 1115 1324 378 ,821/5,22/5,24/5 and 20/7 1157 331 4

B 18/4 to 11/5 1668 477 921/5,22/5 and 19/7 1526 436 3

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K. F. H. MURRELL AND BEL FORSAITH 251

Murrell (1962) had proposed that a break should be given if and when there was a markedchange in the regularity of the performance. If the standard deviation is used as a measureof this effect the majority of the runs showed increases (i.e, greater variability) with time.Graphic examples are given in Fig. 3. Since it is rather tedious to bring together standard

zo~swQ

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~

1.2

SUBJECT A

2

HOURS

Fig. 3. Examples of fluctuation of standard deviation with time.

3

16/4

\\\ 10/5

25{4

11{5

deviations for a number of days in meaningful form Murrell had proposed that a simpler"quality control" technique might be applied to the cycle times by setting an upper limit2 S. Ds. above the mean over the period after warm-up; cycles outside this limit (the longlimes) would then be counted. In the event, examination of the experimental data showedthat it was quite possible for there to be irregularities in the performance unaccompanied byany marked change in the standard deviation so that the "long times" would seem to be amore sensitive indicator of the onset of a disintegration of performance than standarddeviation.

The frequency of occurrence of the long times which result from applying this techniqueto the present data is given in percentage form in Figs. 4a and 5a (the total long times in3t hours = 100%). A showed a marked increase after I hour and again after 2!-hours workwhile B also showed a sharp increase after I hour with the second less well marked at 2thours. Since the two increases are similar for both Ss it is at these points that breaks shouldbe given according to Murrell's proposal.

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REPETITIVE WORK: REPORTS ON TWO SUBJECTS

2

HOURS

Fig. 4. Subject A. Long times, output and misses during continuous conditions.

HOURS

Fig. 5. Subject B. Long times, output and misses in continuous conditions.

b

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K. F. H. MURRELL AND BEL FORSAITH 253

In order to examine the alternative proposal of Wyatt (1927) that breaks should occurjust before output starts to fall, curves of output for each S are given in Figs. 4b and 5b.The most striking feature of these curves is the occurrence of a "low" every 45 mins.Considering the period as a whole there appears to be a slight decline of about I~ %in outputthroughout the work period (due almost entirely to B). In this respect the performance ofour Ss seems to differ from that indicated by the "saddleback" curve which Dudley (1958)examined. It differs also from most of the performance curves given in the various I. H. R.B.reports. Output in industry is subject to a number of disturbing factors which were notpresent in our laboratory; it would seem reasonable to suppose that our Ss showed naturalfluctuations of output uncontaminated by these factors and as a result there was no singlesharp decline in output of the kind used as an indicator by the I.H.R.B. investigators. Atthe time the experiment was going on a quick decision had to be made on when a breakshould occur and this was that a single break should be given at 2 hours based on B's outputcurve; pauses were given at a similar time in a number of the I.H.R.B. experiments. As itnow turns out we probably ought to have tried 4 breaks at 45 min. intervals.

In the determination of the long times, each control limit was based on the mean cycletime of the level performance after the warm-up was complete. Ifin any subsequent quarterhour the mean cycle time was higher than this the whole distribution would be raised and itmight be supposed that more of the longer cycle times would go outside the selected limit.On this basis, if the long times were solely a function of output, there should be an increasein long times whenever there was a decrease in output and for complete agreement the cor­relation between the output and long times curves should approach r = - 1·0. This was notthe case; for A the correlation is r = - 0·153 and for B, r = - O'252; both of these are non­significant. Thus it can be concluded that the long times are a measure of a factor which isindependent of the gross fluctuations in output; there is, however, a general tendency for thenumber of long times to increase towards the end of the working period and this could beassociated with the slight decline in output already noted.

Paced (P.C.)

Ss were tested under various rates of work, the results are given in Table 2.

Table 2. Machine rate and output under P.C.

(data for 3!- hr. shift)

Subject Rate· Output Misses

1209 1122 871275 1100 175

A1325 1139 1861327 1120 2071331 1161 1701342 1063 279

1535 1425 1101544 1464 801547 1483 641548 1471 77

B1562 1441 1211569 1460 1091589 1449 1401690 1442 2481692 1408 2841704 1433 271

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254 REPETITIVE WORK: REPORTS ON TWO SUBJECTS

This table shows very clearly that the number of misses increases with the machine rate;this aspect has been dealt with in the previous issue of this Journal (see p. 169, Murrell,1963). The primary object of this part of the experiment was to find out if changes in thenumber of misses could be used, in the same way as were the long times, as an indication ofperiodic irregularity of performance. The incidence of misses in relation to elapsed time isshown for A in Fig. 4c and for B in Fig. 5c. A gave sharp increases of misses at I hour and2 hours and thereafter oscillated. B also gave a sharp increase at I hour but other increasesare less definite, although a second increase starts at 2 hours. Although the indications of"peaks" in these curves are not very definite, three breaks at I hour, It hours and 2! hourswere introduced into the paced condition.

The fluctuation in the incidence of misses could be due to whatever influence causes thefluctuations of output in the unpaced condition. If this were so there should be a highnegative correlation between misses and output. The opposite is the case, for A, r = +0·545(p < 0,025) and for B, r = - 0·069 (n.s.). On the other hand the correlations betweenmisses in the paced conditions and long times in the unpaced condition are r = 0·526(p < 0-05) and r = 0·730 (p < 0,005) for A and B respectively. These correlations suggestthat the incidence of misses is related to irregularity in the unpaced condition (as revealedby the long times) rather than to any changes in the rate of work. This relationship is not,however, complete. Examination of individual and combined curves shows that the firsttwo increases in the long times in UP.c. are at It and 2! hours, while those of the misses inP.C. are at I and 2 houi s. Murrell (1962) has suggested that the length of the actile periodwill be related to the demand made by the job on the operator. If we accept that paced workis more demanding than unpaced work these results could be taken as showing that theactile period for the paced condition is I hour and that for the unpaced condition is It hourswhich is in accordance with Murrell's postulate.

RESULTS: EFFECT OF INTRODUCING BREAKS

Unpaced work (UP.B.)

In the continuous condition the long times showed marked increases at It and 2t hours.As it was not clear whether the breaks should be given before long times increased or at thepoint of increase, two conditions were tested, with breaks of 5 mins. after I hour and 2thours (UP.2B.I.) and after It and 2! hours (UP.2B.lt). As a third condition one singlebreak of 10 mins. was given at 2 hours (UP.IB.2.). The total outputs achieved under thesethree conditions in 3 hours 20 minutes is given in Table 3 together with the hourly rate ofwork. The breaks were taken as rest. .

Two questions may be tested with these data: (I) what effect did the breaks have upon theoutput and rate of work, and (2) upon the variability.

Output and rate of work

There is some difficulty in answering this question because it is not clear what valueshould be taken for UP.c. performance. It has already been noted that this reached a highlevel and then declined, a decline which was confirmed on later retest. With both Ss the rateof work and the output were higher with breaks than they were in the second part of theUP.C. condition. (UP.c. (1st part) relates to tests given up to 11/5 inclusive. UP.c. (2ndpart) relates to those given on or after 21/5.) Compared with the first part, however, theoutput of both Ss was lower although the rate of work was higher in UP.2B.I. with A andin all the break conditions with B. Owing to the small numbers involved it is not possibleto carry out very meaningful statistical tests on these individual results but the H-test

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K. F. H. MURRELL AND BEL FORSAITH 255

Table 3. Output per shift in unpaced condition with breaks

UP.IB.2.._-----

Rate of Output Rate ofwork work

388 1194 358361 1244 373338 1206 362355 1209 363

361 1213 364

474 1557 467484 1647 494501 1645 493510

------492 1613 484

UP.2B.I].

1640486

393 1294394 1204374 1128

1185

387 1203

475 1581478 1612504 1669

1700

UP.2B.I.

----1----------1619

131113121247

158515931680

-----1-----------Output Rate of Outputwork

------------

B

A

Mean

Mean 1290

Subject

(Kruskal-Wallis one-way analysis of variance) gives p < 0·02 between the rates of work inthe continuous and the three break conditions and p = 0·063 between the outputs in thebreak conditions only, all with A. No other conditions are significant. If the data of bothSs are combined the values obtained are shown in Table 4-with UP.C. (2nd part) = 100.The U-test (Mann-Whitney) has been made on the combined data and the resulting proba­bilities are given for both output and rate of work in Table 5.

Table 4. Combined output and hourly rates relative 10 UP.C. (2nd part)under unpaced conditions

Condition Output Hourly rate

UP.C. (l st part) 114'4 114·4UP.c. (2nd part) 100·0 100·0UP.2B.\. 110·6 116·2UP.2B.I:h 108·1 113·6UP.I B.2. 105'3 110'4

Table 5. One-tailed probabilities for output and hourly rate underunpaced conditions

Condition UP.c. UP.C. UP.2B.1. UP.2B.I]. UP.IB.2.(1st part) (2nd part)

UP.c. (l st part) <-P = 0·001 N.S. <-P < 0·025 <-P ~ 0·001

UP.c. (2nd part) jp = 0·007 jp = 0·027 jp = 0·082

UP.2B.\. <-p < 0·05 <-p = 0·091 <-p = 0·007

UP.2B.I]. N.S. <-p = 0·01 N.S.

UP.IB,2. N.S. <-p ~ 0'006 jp ~ 0·007

vHOURLY RATE

(Note : The arrow points to the condition giving the higher value)

ToTAL

oUTPUT

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256 REPETITIVE WORK: REPORTS ON TWO SUBJECTS

From these two tables the following points can be made. (I) That output under theUP.2B.!. is not significantly different from that under UP.c. (I st part) but is higher thanthat under all other conditions. The output under all three break conditions is significantlyhigher than that under UP.c. (2nd part). (2) That the hourly rate under UP.2B.!. is sig­nificantly higher than that under all other conditions. There is no significant differencebetween the other two break conditions and UP.c. (I st part) but they are significantlyfaster than UP.c. (2nd part). (3) There is no significant difference between UP.2B.lt.and UP.I B.2. in either output or rate.

From this we can conclude that in comparison with UP.C. (1st part) the introduction oftwo breaks at the most favourable times increased the rate of work but not sufficiently tocompensate for lost time and so increased the output. However, when comparison is madeagainst UP.C. (2nd part) all the pause conditions gave both a higher work rate and increasedoutput even though the time worked is reduced by 4·75 %. Under the UP.2B.!. conditionthe rate of work increased 16.2%and output 10·6%.

Variability

In order to investigate the possibility that the introduction of breaks would reduce thevariability of our subjects the pooled variances were calculated and these are given in Table6. Useful though the variance is as a statistic it does not take into account the order inwhich the data from which it is derived are generated. Thus the same variance will befound from the sequence 6,6,6,7,7, 7, 8, 8, 8, as from the sequence 6,8,6,7, 8, 7, 6, 8, 7,yet the performance in thc latter sequence is more irregular than that in the first. With thisin mind comparisons of variability under the various conditions have been made by takingthe mean of the differences (without regard to sign) between successive S.Ds. over the 2ndto the 14th quarter-hour periods and dividing this mean by the mean cycle time of the wholeexperimental session. The resulting "summed differences" and the output data are alsogiven in Table 6 in raw and percentage form.

Table 6. Output in 3+ hours and variability under various conditions

Subject

A

Il

ConditionStatistic

IUP.c.

IUP.2B.1. UP.2B.li. UP.IB.2.

Output' 1157 (100) t290 (111'5) 1203 (104'0) 1213 (104'8)Pooled variance 1'3650 (100) 1'1119 (81.5) 1·2161 (89'1) 1·3433 (98'4)Summed differences (x 104) 133 (100) 126 (94'7) 118 (88'7) 157 (J 18,0)

Output' 1526 (100) 1619 (106'1) 1640 (107'5) 1613 (105'7)Pooled variance 0·4920 (100) 0-4356 (88'5) 0·3499 (71'1) 0'4294 (87'3)Summed differences (x 104) 130 (100) 133 (102.3) 85 (65-4) 144 (108'7)

• outputs are for UP.C. (2nd part) only.

For B alone the H-test gives p < 0,05 for both the pooled variances and summed differ­cnces between the continuous and all the break conditions. The break conditions alone arenot significant, nor are any of A's results. When data from both Ss are combined the onlydifferences in pooled variance which are significant on the Ll-test are UP.2B.I!. < UP.c.at p < 0·025 and UP.2B.1. < UP.C. at p = 0·05. Neither of the two break conditionsdiffers significantly from UP. I B.2. With the summed differences UP.2B.I!. differs sig­nificantly from all the other three conditions (UP.C. at p = 0·01; UP.2B.!. at p = 0·017;UP.! B.2 at P = 0,021). No other differences are significant.

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K. F. H. MURRELL AND BEL FORSAITH 257

These results confirm that the introduction of two pauses into the working period willreduce the variability of the performance.

Paced work (P.B.)

Breaks of 5 mins. each were given after I, It and 2-\- hours work and the total timeworked was 3 hrs. 15 mins. These breaks were possibly not at the best times; the curves ofmisses made by the two Ss differ slightly but the times chosen seemed to be the best com­promise. In order to compare the results when 3 breaks were given with those from the P.c.condition (each session of which lasted 3 hrs. 30 mins.) the data obtained are shown againstthe machine rate and are given in Table 7 with the nearest comparable sessions for B whenno breaks were given. Unfortunately, the only comparable session for A is at 383. It ispossible to estimate the probable misses at the other two speeds from a curve fitted to theavailable data by a method described in Murrell (1963) and these are given for comparison.

Table 7. "Machine rate", output and misses for paced work withand without breaks

c3P. B.1. P.

Hourly Output Misses Misses as Hourly Output Misses Misses asrate % rate rate % rate

373 1052 160 13·2 373 1103 202 15·4"383 1074 171 13'7 383 1063 279 26'3388 1090 172 13·6 388 1063 295 27'7"

480 1507 53 3'3 483 1442 248 14'7484 1529 44 2-9 483 1408 284 16·8

S

B

A

" estimated data

It will be seen that the data are rather sparse, with only three successful sessions for Aand two for B. A appears to have been less influenced by the breaks than was B. There arereductions in her misses but they are sufficient to compensate for the loss of 15mins. workingtime at only the two faster rates.

With B the results were startlingly different (so different, in fact, that her records have beenrechecked twice) the effect of introducing breaks being to reduce the misses to less than afifth of the number when work was continuous with a consequence increase of averageactual output from 1425 to 1504 (5,5%) in spite of working 15 mins.less. No useful statisti­cal tests can be carried out on these data and so the best that can be said is that the intro­duction of breaks can lead to an increase in output but that nothing indicates the reasonwhy there should be a substantial increase with one subject and little with the other.

Subjects' knowledge of results

Throughout any day's work the Ss were not told what their output was, nor was there anymeans by which they could tell the time. This arrangement was adopted because it wasthought that the Ss might deliberately influence the results of the experiment. However,towards the end of the experiment, B was told that the best that she had done previously wasabout 1700 under UP.c. and on three days under UP.2B.1. she was told her output everyquarter hour compared with that required to exceed this target. She achieved 1806,1808 and1831 a performance which is 5 %better than the best previously achieved under the UP.C.

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258 REPETITIVE WORK: REPORTS ON TWO SUBJECTS

condition and 12% better than the average of UP.2B.!. run a fortnight before. It has longbeen known that knowledge of results can improve output on a repetitive task so thisresult is not very surprising. It is, however, evidence additional to that given earlier in thisreport that the level of performance which was adopted after the first couple of weeks testingwas below that which was possible. B showed only small changes in rate of work throughouta normal session under UP.2B.!. but when she was given knowledge of results and wasprobably making a real effort she was somewhat less regular and showed a decline inoutput of 8 % during the last hour.

DISCUSSION

Unpaced work

Within the limitations of an experiment with two Ss it would appear that the results showperiodicities not usually found in industrial experiments. Output (or rate of work) seems tobe fluctuating on a 45 mins. cycle, while irregularity in performance, in so far as it is revealedby the "long times", runs on a 75 mins, cycle. The latter result had been anticipated in theideas leading to this research but the former was quite unexpected. These periodicities seemto show that the rate of work and irregularity in the work are independent of each other andit remains to be seen which can best be used as an indication of the number and location ofbreaks in the work.

It has been assumed (following the work of the !.F.R.B.) that some break is required in along period of continuous work and that under industrial conditions this will lead toincreased output, so it is not unduly worrying to find that compared with UP.C. (1st part)output declined when rest was given. On the other hand comparisons between outputunder UP.c. (2nd part) and the break conditions show that a 2-break condition gavegreater output than I-break or no break. Comparisons based on variability are moreconclusive. When two breaks were given performance was more regular than either UP.c.or UP.I B.2. These two subjects, therefore, confirm the proposal that breaks given at thepoint in time when work becomes more irregular can result in better performance thanwill be obtained when breaks are given arbitrarily or are based on' output. It must beemphasised that absolutely 110 conclusions can be drawnfrom the data obtained ill this experi­ment as to the magnitude of any increase in output which might result from the introductionofbreaks ill industry.

From the data in Table 6 it appears that A performed best under UP.2B.I. on two of thethree measures used and BundeI' UP.2B.I!. on all three. Although not all these differencesare significant the combined differences on different criteria suggest the possibility that,although they both gave increases in long times during the fifth 15 mins., the subjects shouldrest at slightly different times which might have become more evident had a scoring intervalof less than 15 mins. been used. Their work curves seems to support this idea. During thelast hour A's rate of work declined rather less under U P.2B.!. (386-384) than underUP.2B.1t. (361-354) while B's rises under UP.2B.I!. (492-502) but declines under UP.2B.I.(488-480). B was the faster and steadier worker and she may well have found the work lessof an effort than A who had had less previous experience of repetitive factory work. Ifthis were the case, and the suggestion is correct that the interval before a rest is required willdepend on the individual, then A's better performance with the earlier rest could be ex­plained.

It may be asked whether the long times are a better indication of a change in perfor­mance than the variance. In order to shed some light on this question the pooled variancesfor each 15 mins. have been computed for each S. These are given with the superimposedlong times in Fig. 6. Up to It hours the curves are very similar, including the sharp rise

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K. F. H. MURRELL AND BEL FORSAITH

SUBJECT B

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Fig. 6. Comparison of variance with long times.

12

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VARIANCE

-._-_ LONG TIMES

.- -_.- ....

SUBJECT A

after I hour but here the similarity ends. A's curve shows a very sharp rise after 21 hourswhile B's curve is irregular and indeterminate. If these variance curves were to be used todetermine when breaks are given B's would be useless beyond the first break. A's curvewould seem to indicate that she should have breaks similar to those given under UP.2B.lt.on which she performed less well. This experiment, therefore, does not give any evidencethat breaks can be better determined by using changes in variance rather than long times.

There is difficulty in using these variance curves. In industry it is not practicable to havebreaks at different times for different individuals and in any event it is not likely thatindividual differences of any great consequence will be found on any particular job providedsuitable operatives are employed. Therefore, to be practically useful it must be possihle tobring together data from more than one operative. This cannot be done with the variancecurves of the two Ss ; the correlation between them is r = -0,258 (n.s.) and together theygive a combined curve which is meaningless. On the other hand the correlation between thelong times curves is r = 0·632 (p < 0,01) and when combined they give a curve from whicha useful conclusion can be drawn (Fig. 7).

It must be remembered that the object is not to measure variability as such but to deter­mine the points at which a disintegration in performance commences. The evidence suggeststhat the technique of long times did this adequately for these two subjects.

Paced work

During continuous paced work the number of misses seems to have increased at hourlyintervals and is probably not related to the rate of work in the unpaced condition but to the

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260 REPETITIVE WORK: REPORTS ON TWO SUBJECTS

2

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Fig. 7. Both subjects. Long times in continuous conditions.

3

variability, although the actile period was 15 mins. shorter than in the unpaced condition.Paced work is more stressful than unpaced so this shorter period is in accordance withhypothesis.

In four out of the five sessions the introduction of three breaks at the times at which misseshad been found to increase gave an output which was equal to or above that obtained underP.C. This confirms the findings of the I.F.R.B. but it does not, of itself, show that threebreaks arc the optimum number. We had hoped to try both two and one break sessions buttime ran out on us. It is possible to compute from B's results that the hourly rate can beincreased from 368 and 388 when three breaks are introduced without increasing the numberof misses. The output is, however, somewhat reduced. With A, on the other hand, rate ofwork can be increased from 431 to 482 with a corresponding increase in output from 1465to 1515 without increasing the number of misses.

SUPPLEMENTARY INFORMATION FROM THE EXPERIMENT

Physiological measurement

It has been one of the assumptions underlying this project that the changes in performanceshown by the subjects are central in origin rather than peripheral. However, the notion thatpeople on light repetitive work become "fatigued" in the muscular sense (and require a"fatigue" allowance) dies hard in industry and so we decided to make some recordingswith an electromyograph. Surface electrodes were placed on the forearm just below theelbow, as near as possible on flexor digitorum superficialis and brachioradialis with anearth on extensor digitorum. Records were taken at 15 min. intervals throughout severalUP.C. sessions. At the conclusion ofa session the subjects were required to crank (against africtional load of 8 lb.) to exhaustion. After 15 sees. pause they continued the normal taskfor ten cycles.

We were unable to detect any differences between traces made at successive 15 min.intervals nor between those made at the beginning and the end of a session (e.g, one of as'straces is shown in Fig. 8). There was, however, a very marked difference between the tracestaken before and after cranking (Fig. 9). Since the latter are from muscles known to befatigued it is not unreasonable to assume that there is no noticeable fatigue at the end of a3! hour session.

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K. F. H. MURRELL AND BEL FORSAITH 261

a

bFig. 8. Electromyographs from subject A. (a) at end of 15 ruins. work (b) after 3! hours work.

a

bFig. 9. Electromyographs from subject B. (a) after 3~ hours work (b) after cranking to exhaustion.

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262 REPETITIVE WORK: REPORTS ON TWO SUBJECTS

Additional evidence comes from the various performance curves. Under UP.c. theoutput and long times are completely out of phase and thus are unlikely to have a commonsimple cause. Moreover, the output curves do not show a sharp decline towards the endof the session which might be expected were muscular fatigue present, except perhaps underUP.2B.1. (with knowledge of results) when subject B may have been working close to herlimit.

Although the evidence here presented is negative rather than positive it docs seem tosupport the view that the variations in performance found in this experiment are notperipheral in origin.

Method 0/ determininq "lonq limes"

The method which was proposed for scoring the long times (Murrell 1962) proved to bedifficult to apply because of uncertainty as to which means and standard deviations wereto be included in the initial level performance. An alternative and simpler method wasdevised in which the longest cycle time in each quarter of an hour was examined and theupper limit was set at the shortest of these. For example, on 10/5/62 subject A gave thefollowing longest duration of cycle times in successive quarter hours: 12·2; 11·2; 12·2;11'8; 12·6; 13·6; 13·0; 12·6; 12·8; 12·0; 12'0; 12-6; 12·6 sees, The lowest of these values is11'2 sees. and this is, therefore, taken as the upper limit and all longer cycles are counted as

.SUBJECT A

8

6 • ;---- .......",' ' -,

;~

~4· .,,-_ ;'::t " --.... "",~ ,," ----.._-.... ,---- .... /~ 2--- -- - - .. ,,,,o....

I31

,---- .... ""-- - .. I""'"

'1

1 L-. --=.r__~ ~___L _!:_---

SUBJECT B

HOURS

Fig. 10. Long times obtained from 2 S.D. (solid Jines) and least times (broken lines).

long times. Comparisons between the results of the two methods are shown in Fig. 10which indicates that for the purpose for which the long times are required the alternativemethod seems to be satisfactory.

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K. F. H. MURRELL AND BEL FORSAITH 2(,)

CONCLUSION

These preliminary results with two subjects agree with the various proposals put forwardby Murrell (1962) which were deduced from the work of others. Although they must notbe taken as complete confirmation they do suggest that the proposals have some merit andwarrant continued investigation.

ACKNOWLEDGEMENTS

Barbara Spencer-Jones and Michael Kingston have been of great help by acting as reliefsupervision and by assisting with the computations, while Peter Powesland and JohnSpencer have read and made detailed comments upon this report. We are very grateful toall of them. This research is part of a programme supported by a grant from thc Depart­ment of Scientific and Industrial Research.

Cet article constitue Ie premier d'une serie, entreprise afin d'cxaminer Ics hypotheses avanceespar Ie premier auteur selon lesquelles, dans Ie travail ayant des elements cycliques: (i) Ierendcment serait plus grand, et la variabilite serait moindre, s'il y avail un temps de fCPOS alafin d'une peri ode actile (periode de rendement optimum) plutot que lorsque Ie rendementdecroit ; (ii) la fin d'une periode actile serait signalee par I'apparition de rendements irreguliers,qui pourraient etre determines par l'incidence de longues durees du cycle dans Ie travail libre,et par Ie nombre de cycles rnanques dans Ie travail it une allure determinee ; (iii) la duree d'uneperiode actile dependra de l'exigence de la tache el de la capacite de l'ouvriere d'y faire face(son actilite),

Des essais conduits sur deux sujets pendant 3.1/2 mois semblent montrer que l'hypothcse (i) estcorrecte. Lorsque deux peri odes de repos furent prises aussitot que les periodes, au travaillibre, s'allongerent, Ie rendement s'ameliora sensiblement et it y avait moins de variance quelorsqu' une peri ode de repos fut prise au moment ou se produisit une baisse dans Ie rendementTous les deux sujets faisaient rnoins de fautes dans Ie travail libre lorsque trois periodes derepos furent donnees, qu'au travail continuo

La technique pour determiner de longues peri odes des elements cycliques au travail libre,qui faisait partie de l'hypothese (ii) fut trouvee praticable et une methode plus simple fut miseau point. Les resultats sernblent confirrner que la methode est valable. On ne peut pas tirerde conclusions valables du travail a une allure deterrninee.

L'hypothese (iii) est partiellement confirrnee ; it une allure deterrninee la periode actile etaitd'une heure, comparee avec 1.1/4 heures au travail Iibre. En plus. la meilleure ouvriere faisaitmeilleur rendement lorsque la periode de repos fut prise apres 1.1/4 heures, tandis que la moinsbonne ouvriere donna de son mieux lorsqu'elle se reposa apres une heure.

Diese Arbeit stellt die erste einer Reihe von Versuchen dar, die angestellt wurden urn einigevom Hauptverfasser fiir die serienarbeit autgestellte Hypothesen zu prufen, namlich: (i) dieProduktion steigt, und die Variabilitiit fallt, sobald eine Pause am Ende einer uktilen Periode(Optimalleistungszeit) angestellt wird, als wenn man wartet bis die Produktion abfallt ; (ii) dusEnde einer aktilen Periode wird dadurch erkannt, dass Unregelmassigkeiten entstehen, welchcsich durch Vorkommen von langen Arbeitstaktzeiten wahrend Arbeit mit Schrittmachung,und durch Anstieg verfehlter Arbeitstakte bei Arbeit ohne Schrittrnachung, bemerkbar machen;(iii) die Uinge der aktilen Periode hangt ab von den Anspriichen, die die Arbcit aufdie Arbeiterinrnacht, sowie ihre Fahigkeit diesen nachzukommen (ihre Aktilitat).

Die wiihrend 3.1/2 Monaten an 2 Versuchspersonen angestellten Versuche zeigen, dassHypothese (i) die richtige ist. Wurden 2 Ruhepausen gemacht sobald, ohne Schrittrnachung,die Arbeitszeiten sich verlangerten, so ergab sich eine wesentlich bessere Produktion, mit einerniedrigeren Varianz, als wenn erst dann eine Pause zu dem Zeitpunkt gemacht wurde, als dieProduktion fiel. Beide Versuchspersonen begingen relativ weniger Fehler wenn, ohne Schritt­machung, 3 Pausen gemacht wurden, als wenn sie ununterbrochen arbeiteten.

Das Verfahren urn die langen Arbeitstaktzeiten bei Arbeit ohne Schrittrnachung zu rnessen,mittels dessen Hypothese (ii) gepruft wurde, erwies sich als praktisch, und eine einfachereMethode wurde entworfen. Die Resultate scheinen die Gultigkeit der Methode zu bestiitigen.Uber Arbeit mit Schrittmachung konnen keine endgultige Schlussfolgerungen gezogen werden.

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264 REPETITIVE WORK: REPORTS ON TWO SUBJECTS

Hypothese (iii) scheint partiell bestatigt zu sein; mit Schrittmachung war die AktilperiodeI Stunde, verglichen mit 1,1/4 Stunde ohne Schrittmachung. Fernererzieltedie bessereArbeiterinihre beste Leisung, wenn sic nach 1.1/4 Stunden eine Pause machte, wiihrend die schlechtereArbeiterin am besten arbeitete, wenn sie nach einer Stunde ruhte,

REFERENCES

DUDLEY, N. A.(1958). Output pattern in repetitive tasks. lnst, Prod. Enqrs. J., 37,187.MUkRELL, K. F. H. (1962). Operator variability and its industrial consequences. 1111. Jill. Prod. Res., I,

No.3,39.MURRELL, K. F. H. (1963). Laboratory studies of repetitive work: I: Paced work and its relation to unpaced

work. 1111. Jill. Prod. Res., 2, No.3, 169.WYAIT, S. (1927). Re.I·1 pauses ill industry, I.F.R.B. Report, No. 42, London: H.M.S.O.

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