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Rotating-shift system vs. fixed-shift system
International Journal of Industrial Ergonomics, 7 (1991) 63-70 Elsevier
63
Case study
Rotating-shift system vs. fixed-shift system T i a n - S h y L i o u a n d M a o - J i u n J. W a n g Department of lndustrial Engineering, National Tsing Hua University, Hsin-Chu, Taiwan, ROC
(Received April 4, 1990; accepted in revised form June 4, 1990)
Abstract The performances of two different shift work systems were assessed in a resistor manufacturing company over a period of 13 months. At first, the existing rotating-shift system was evaluated; the yield and productivity were found to be significantly affected by shift and working day factors ( p < 0.05). The day shift had the best performance followed by swing and night shift. Both the yield and productivity rose for the first four working days, then declined for the following two days. Subsequently, a new fixed-shift system was proposed and the performances again were evaluated. Once again, the shift effect was found to be significant for both productivity and yield. Comparing the differences between the two shift systems, both productivity and yield have shown significant differences in swing and day shift (p <0.05). For the night shift, performance difference between the rotating-shift and the fixed-shift systems was found at p < 0.1. In summary, the fixed-shift system was found to be superior to the rotating-shift system in many aspects. A few months after implementing the fixed-shift system, the company decid-.d to return to the rotating-shift system due to the high turnovers involved in the night-shift workers. The work shift system design implication is also discussed.
Relevance to industry This paper presents the finding of evaluating two work-shift systems in a resistor manufacturing company. Since this is a real case study, it is obvious that the work is directly relevant to industry. Keywords Rotating shift, fixed shift, yield, productivity, circadian rhythms, sleep deprivation.
1. Introduction Shift w o r k is d e f i n e d as w o r k i n g o t h e r t h a n d a y t i m e hours. T h e system of three-shift r o t a t i o n s h a s been d e v e l o p e d since the 1920s (Sergean, 1971). T h e early reasons for e x t e n d i n g h o u r s b e y o n d the d a y t i m e shift were b a s e d o n a need for continuous services o r c o n t i n u o u s o p e r a t i o n s , for exa m p l e , g e n e r a t i n g electrical utilities o r c h e m i c a l m a n u f a c t u r i n g processes. In recent years, due to r a p i d changes in t e c h n o l o g y a n d increased g l o b a l c o m p e t i t i o n in the m a r k e t place, the g r o w i n g a p p l i c a t i o n s o f a shift w o r k system have b e e n l i n k e d to the e c o n o m i c s o f m a n u f a c t u r i n g . By using the e q u i p m e n t a r o u n d the clock, the c o m p a n y can 0169-1936/91/$03.50 © 1991 - Elsevier Science Publishers B.V.
recover the c a p i t a l i n v e s t m e n t m u c h earlier t h a n the p r o d u c t ' s life cycle ( E a s t m a n K o d a k , 1983). However, research has i n d i c a t e d that shift work, a n d night w o r k in p a r t i c u l a r , m a y have a n a d v e r s e effect on the h e a l t h a n d w e l l - b e i n g of the w o r k e r s ( J o h n s o n et al., 1981; R u t e n f r a n z et al., 1977). This is a t t r i b u t e d to the fact t h a t w o r k e r s have to w o r k at times which are d i s r u p t i v e to their p h y s i o logical, p s y c h o l o g i c a l a n d social c i r c a d i a n r h y t h m s (Aschoff, 1981; Wever, 1981). T h e effect is m a n i fested not o n l y in d a i l y life b e h a v i o r b u t also in j o b p e r f o r m a n c e . U s u a l l y the j o b p e r f o r m a n c e s e v a l u a t e d in shift w o r k studies were p r o d u c t i v i t y , e r r o r rate a n d a c c i d e n t r a t e ( E a s t m a n K o d a k , 1983). T h e shift effect o n a c c i d e n t rate has b e e n s t u d i e d b y various investigators, b u t n o c o n s i s t e n t
Tian-Shy Liou, Mao-Jiun J. Wang / Rotating- t's. fixed-shift system
64
d Start -I the capping machine
findings have been reported (Vernon, 1920; Wyatt and Marriott, 1953; Brandt, 1969). In addition, performance evaluation based on accident rate tends to be less representative when the observation period is not long enough. Many types of shift work systems are available. The selection of a suitable shift system has to consider the production needs, the job requirements (physical or mental), and the psychological consequences of the schedule. General guidelines for the design and evaluation of shift schedules are available in various human factors references (e.g. Eastman Kodak, 1983; Smith, 1987). In this study, the performance of a rotating-shift work system in a resistor manufacturing company has been evaluated. Subsequently, a new fixed-shift work system was proposed and the performance measures in terms of productivity and yield were further evaluated.
L I vibration AO,and s,,o J L IMake sure capping math speed
r
ake appropriate~ actions |
~-ine operate smooth y
I theOe,erm,oe I I causes
,osoect ]
the capped Ros
and check quantity
2. Problem situation
The study was conducted in an electronics company which produces resistors. There were 180 employees. Five major operations are involved in the resistors manufacturing process: roding, capping, helixing, cameling, and packaging automatically by a capping machine. The Ro is a ceramic rod after electroless nickel plating or magnetron sputtering. The operator's main task is to feed caps and Ros into the capping machine, to monitor the process, and to inspect the capped Ros. Each capped Ro consists of one Ro and two caps. The capped Ro is considered to be defective if only one cap or no cap was assembled. The description of the capping operation is shown in figure 1. The operation was originally conducted under a rotating-shift system which is a rotating hours and continuous work week system. Each
I subsequent Oet"Ooperation 'o I Fig. t. Task description of the capping operation.
operator worked six consecutive days, and then took two days off. A total of 24 operators was involved in the capping operation, and they were all males. The mean age was 29 years, ranging from 24 to 37 years. The average on the job experience was 4.2 years (range from 1.2 to 13 years). The 24 operators were divided into four groups with six workers in each group. The shifts rotated in a sequence of day, swing, and night. The three shift schedules were 07.30 to 16.00 h for day, 15.30 to
Table 1 Work schedule for a rotating-shift cycle (24 days complete a shift cycle). Day Group Group Group Group
1 2 3 4
M
T
W
Th
F
S
Su
M
T
W
Th
F
S
Su
M
T
W
Th
F
S
Su
M
T
W
D R S N
D R S N
D S R N
D S R N
D S N R
D S N R
R S N D
R S N D
S R N D
S R N D
S N R D
S N R D
S N D R
S N D R
R N D S
R N D S
N R D S
N R D S
N D R S
N D R S
N D S R
N D S R
R D S N
R D S N
D = day: S = swing: N = night; R = rest.
Tian-Shy Liou, Mao-Jiun J. Wang / Rotating- cs. fixed-shift system 24.00 h for swing, a n d 23.30 to 08.00 h for night. O n e r o t a t i n g cycle takes 24 d a y s a n d the schedule is d e m o n s t r a t e d in table 1. Each o p e r a t o r h a n d l e d two c a p p i n g m a c h i n e s s i m u l t a n e o u s l y ; the machine were not always a t t e n d e d b y the s a m e o p e r ator. A total of 12 c a p p i n g m a c h i n e s is in o p e r a t i o n d u r i n g each shift.
3. The rotating-shift system 3.1. Performance evaluation o f the rotating-shift system F o r the r o t a t i n g - s h i f t system, p e r f o r m a n c e s were collected for a p e r i o d o f 192 d a y s which c o v e r e d eight shift cycles. A n e x p e r i m e n t analysis was a p p l i e d to e v a l u a t e factors affecting j o b performances. T h e i n d e p e n d e n t variables were w o r k ing g r o u p (4 groups), shift (3 levels: day, swing, a n d night), w o r k i n g d a y (6 days), a n d o p e r a t o r (6 people). T h e eight shift cycles were c o n s i d e r e d as eight replications in each c o n d i t i o n . T h e perform a n c e measures were yield a n d p r o d u c t i v i t y . T h e y i e l d was defined as the ratio b e t w e e n o u t p u t a n d i n p u t in percentage. T h e i n p u t a n d o u t p u t were m e a s u r e d b y a specific electronic b a l a n c e with ' p i e c e s ' as its unit. T h e p r o d u c t i v i t y (with n u m b e r of pieces p e r h o u r as its unit) was d e f i n e d as the r a t i o of o u t p u t to the actual o p e r a t i o n time, which excludes a m a c h i n e ' s d o w n time, a n d idle time d u e to insufficient m a t e r i a l supplies. T h e i n p u t a n d o u t p u t were r e c o r d e d b y the o p e r a t o r s a n d c h e c k e d b y the s t o c k m a n . D a i l y p r o d u c t i v i t y a n d yield were p o s t e d on the next day. T h e p e r f o r m a n c e d a t a were a n a l y z e d b y SAS, which is a statistical analysis software. 3.2. Results and discussion T w o f o u r - w a y analyses o f variances were perf o r m e d on yield a n d p r o d u c t i v i t y respectively. T h e results are p r e s e n t e d in tables 2 a n d 3. F o r the m a i n effects, the shift a n d w o r k i n g d a y factors have d e m o n s t r a t e d significant influences (p < 0.001) on b o t h yield a n d p r o d u c t i v i t y . But the w o r k i n g g r o u p a n d o p e r a t o r factors were not significant. T h e lack o f i n d i v i d u a l differences is d u e to the fact that the o p e r a t i o n was p a c e d b y the machine. All of t h e m were e x p e r i e n c e d o p e r a t o r s
65
Table 2 Analysis of variance for yield under the rotating-shift system. Factor
df
Mean square
F value
Group (G) Shift (S) Day (D) Operator (O) G×S G×D G×O Sx D Sx O Dx O G x S× D GxSxO GxDxO Sx D x O GxSxDxO Error
3 2 5 5 6 15 15 10 10 25 30 30 75 50 150 3024
0.07 633.10 91.47 0.03
1.64 13102.11 a 1839.16 a 0.69 0.05 0.04 0.85 32.52 ~ 0.04 0.03 0.05 0.04 0.03 0.04 0.04
0.04 1.57
0.04
Note: The data is truncated to the second digit after the decimal point, a: Significant at p < 0.001; - : < 0.01.
w h o h a d fairly close skill levels, a n d were p a i d b y m o n t h l y salary. R e g a r d i n g i n t e r a c t i o n effects, o n l y the shift × w o r k i n g d a y factor was significant (p < 0.001) for b o t h p r o d u c t i v i t y a n d yield. T h e rest of the i n t e r a c t i o n s were not significant. T h e significant t w o - w a y i n t e r a c t i o n (shift × w o r k i n g d a y ) for b o t h yield a n d p r o d u c t i v i t y is illustrated in figures 2 a n d 3 respectively. W e see that the d a y
Table 3 Analysis of variance for productivity under the rotating-shift system. Factor
df
Mean square
F value
Group (G) Shift (S) Day (D) Operator (O) G×S G×D GxO Sx D Sx O DxO G x Sx D G x Sx O Gx Dx O Sx D x O G×S×D×O Error
3 2 5 5 6 15 15 10 10 25 30 30 75 50 150 3024
39.02 814714.08 116635.20 6.18 1.69 0.84 25.60 1857.53 1.62 1.59 1.85 1.80 1.72 2.45 1.87 53.03
0.74 15363.09 a 2199.39 a 0.12 0.03 0.02 0.48 35.03" 0.03 0.03 0.03 0.03 0.03 0.05 0.04
": Significant at p < 0.001.
Tian-Shy 1.iou, MaoJiun J. Wang / Rotating- vs. fixed-shift system
66 99
98
o
,--I
97
z<
96
95
I
I
I
1
2
3
'"
I
I
4
5
""
I
6
WORKING DAY Fig. 2. Mean yield for the three shifts across six working days under the rotating-shift system.
shift had the best performance followed by swing and night shifts for both yield and productivity. One immediate difference is that more supervisors and managers were around in the day-shift than in the night-shift. In addition, the major contributing factor is due to variation in the effect of rhythmic changes in physiological functions during a 24hour period, with performances dropping during the night, rising during the day, and reaching a peak in the afternoon. The rhythmic changes in physiological functions have been found to be
associated with changes in performance (Monk and Embrey, 1981). Further, Tepas et al. (1985) investigated sleep lengths of the shift workers and reported that night-shift workers slept the least among the three shift workers. Thus performance of the night-shift workers may suffer not only from workers being at a low ebb of the circadian cycle, but also from sleep deprivation effect. Further, performance was found to reach a peak on the fourth working day, declining again on the fifth and sixth days, with the first working day
8400
8200
8000 I'-"
O
A
SWING
7800
t-~
m =ooo,. 0.. z< uJ
.,~
7600
7400
7200
I
I
1
2
"""
I
I
I
I
3
4
5
6
WORKING DAY
Fig. 3. Mean productivity for the three shifts across six working days under the rotating-shift system.
67
Tian-Shy Liou. Mao-Jiun J. Wang / Rotating- vs. fixed-shift system
Table 5 Analysis of variances for yield and productivity under the fixed-shift system.
being the lowest. This is again attributed to the fact that the operator's circadian rhythm was unadjusted after two days off. The unadjustment tends to improve gradually and approaches a peak performance on the fourth working day of a week. It is possible that due to the cumulative fatigue effect the performances dropped during the last two days. Furthermore, a Pearson correlation was conducted between productivity and yield, and a significant positive correlation ( r - - 0 . 9 2 8 , p < 0.001) was found. This indicates that no trade-off effect was found between speed (productivity) and accuracy (yield) performances. The shift and working day factors had a significant influence on both speed and accuracy performances.
Factor
Shift Error
df
2 357
Yield Mean square 79.08 0.11
Productivity Mean F value square
F value 6.92.18a
137 442.03 185.59
740.56 a
Note: The data is truncated to the second digit after the decimal point, a: significant at p < 0.001.
ble for evaluation; 120 replications were involved in each shift level. The dependent variables were again the productivity and yield. 4.2. Result and discussion
4. The fixed-shift system The one-way analyses of variances were performed, and the results are shown in table 5. The shift effect was again found to be significant ( p < 0.001) for both productivity and yield. The average performances in terms of productivity and yield for the three shifts are demonstrated in figures 4 and 5. As expected, the day-shift had the best performance, followed by swing-shift and night-shift. In addition to the previously mentioned reasons of circadian rhythmic adjustments and sleep deprivations, Rosa and Colligan (1989) also reported that the self-rating stress of the day workers was perceived to be the highest a m o n g the three groups, and that of the night workers was the lowest. The workers in this study also indicated that they paid more attention to the j o b in the day-shift than in the night-shift due to the fact that more supervisors and managers were around in the day-shift than in the night-shift. Perhaps the stress due to a close supervision was also a contributing factor to the increased performances of the day-shift workers. Furthermore, the result of
4.1. Performance evaluation of the fixed-shift system
After evaluating the rotating-shift system, a fixed-shift working system was subsequently proposed for evaluation. In the new system, the 24 shift workers were divided into three groups (i.e. day, swing, and night) with eight workers in each group. Similar to the rotating-shift system, each operator worked six consecutive days, and then took two days off. Also within each shift group, only six out of eight workers were on duty, and the remaining two workers were off. The detailed arrangement for the fixed-shift system is shown in table 4. Since the fixed-shift system was a newly adapted system, a two-month adjustment period was given to help the workers get used to the new working schedule. Afterwards, performances over 120 days (15 cycles) were collected. Due to the lack of significant effects on group and operator factors found in the rotating shift system, only the shift factor was utilized as the independent variaTable 4
Work and rest schedule for a fixed-shift cycle. The arabic numerals represent the workers (8 days complete a shift cycle). Day Rest operators Operator from work to rest Operator from rest to work
M 1,2
T 2,3
W 3,4
Th 4,5
F 5,6
S 6,7
Su 7,8
M 8,1
2
3
4
5
6
7
8
1
8
1
2
3
4
5
6
7
68
Tian-Shy Liou, Mao-Jiun J. Wang / Rotating- vs. fixed-shifi system 99 FIXED-SHIFT
q w
98
>Z
< ILl
~E 97 ¸
96
I
!
I
day
swing
night
WORK SHIFT
Fig. 4. Mean yield for each shift under the fixed-shift and rotating-shift system respectively. Pearson correlation between yield and productivity indicated that a significant positive correlation ( r = 0.851, p < 0.001) was found. This again reveals the non-existence of speed and accuracy trade-off effect. Both productivity and yield were significantly affected by the shift factor.
5. Comparison of the two systems In order to evaluate the performance differences between the rotating-shift and the fixed-
shift system, six paired t-tests were conducted. The results are summarized in table 6. Significant differences were found on productivity and yield ( p <0.05) between the rotating-shift and the fixed-shift system in both swing and day shifts. As for the night shift, significant performance differences were found at p < 0.10. Figures 4 and 5 demonstrate the mean performances of the three shifts under the two shift work systems. It is obvious that the fixed-shift system is superior to the rotating-shift system in both productivity and yield. The superiority of the fixed-shift system is
8600 FIXED-SHIFT
8400 A
_> .¢: (.) ca
8200
8I I " _ = ca
8000
7800
7600
Day
Swing
Night
WORKSHIFT Fig. 5. Mean productivity for each shift under the fixed shift and the rotating-shift system respectively.
Tian-Shy Liou, Mao-Jiun J. Wang / Rotating- vs. fixed-shift system
69
Table 6 Significant differences on mean yield and mean productivity between the rotating-shift and the fixed-shift systems under day, swing, and night shift. Fixed-shift
Rotating-shift
t-value
Mean yield (%) Day
M
SD Swing
M
SD Night
M
SD
98.36 0.23 97.66 0.28 96.84 0.28
M
SD M
SD M
SD
97.25 0.49 96.95 0.48 96.24 0.49
2.37 ~
8136.90 148.45 7835.90 119.79 7611.60 151.45
2.72 "
1.86 b 1.48 ~"
Mean productivity ( p c s / h ) Day
M
SD Swing
M
SD Night
M
SD
8450.30 67.54 8127.50 89.20 7800.40 103.27
M
SD M
SD M
SD
2.76 a 1.53 -
Note: df = 310; a: significant at p < 0.01; b: significant at p < 0.05; +: significant at p < 0.1.
not confounded with time because all the operators had at least one year on the job. Therefore, the possible involvement of learning effect is very slim. Further, it is interesting to note that all the standard deviations in the fixed-shift performances are smaller than that of the rotating-shift performances. This indicates the fixed-shift system had more consistent performances than the rotating-shift system. Overall, the fixed-shift system has demonstrated a significantly better performance than the rotating-shift system in many aspects. This may be attributed to the fact that workers who work under the fixed schedule have less circadian rhythm adjustment problems than the rotating-shift workers. Furthermore, it is interesting to find that the performance on the fourth working day of a week in the rotating-shift system is very close to the average performance of the fixed-shift system (table 6 and figures 2 and 3). In other words, the peak performance in the rotating-shift system is about the same as the average performance of the fixed-shift system. Based on these findings, it seems that it is logical for the company to continue implementing the fixed-shift system.
6. Concluding remarks In summary, through 13 months' evaluation, the result of this study indicated that performances in terms of productivity and yield of the fixed-shift schedule are superior to those of the rotating-shift schedule. From interviews with operators it appeared that most of them felt that the fixed-shift system was better, but most of them prefer not to work at night. By providing an appropriate incentive, an increase of about 30% on the day shift salary, the company was able to attract operators to work at night under the permanent hour schedule. On the other hand, it was suggested that after implementing the system the company should also systematically evaluate the additional incentive cost, and the possible detrimental effects due to sleep deprivation and rhythmic changes of the night workers. After a few months' observations, the company decided to return to the rotating-shift system, due to the high turnovers involved in the night shift workers. Even though the company has provided some incentives to the night workers, the company
70
Tian-Shy Liou. Mao-Jiun J. Wang / Rotating- vs. fixed-shift system
still f o u n d it d i f f i c u l t to keep t h e m a n d to recruit n e w w o r k e r s for t h e n i g h t shift o p e r a t i o n . O n e p o s s i b l e e x p l a n a t i o n is that the j o b was d o n e by m a l e o p e r a t o r s , w h o c a n easily find a j o b e l s e w h e r e w i t h o u t s u f f e r i n g d i s r u p t i o n s to their r e g u l a r social lives a n d p h y s i o l o g i c a l r h y t h m s .
Acknowledgments T h e a u t h o r s wish to a c k n o w l e d g e w i t h g r a t i t u d e the c o o p e r a t i o n a n d s u p p o r t o f the staffs, m a n a g e m e n t s a n d shift w o r k e r s at Y o n g t a r e s i s t o r m a n u f a c t u r i n g c o m p a n y at K a o h s u n g , T a i w a n .
References Aschoff. J.. 1981. Circadian rhythms: Interference with and dependence on work-rest schedules. In: L.C. Johnson, D.I. Tapas. W.P. Colquhoun and M.J. Colligan (Eds.), The Twenty-Four Hour Workday: Proceedings of a Symposium on Variation in Work-Sleep Schedules. DHHS (NIOSH) Publication No. 81-127. US Government Printing Office, Washington, DC, pp. 13-50. Brandt. A., 1969. Influence of shift work on the health and pathology of workers. Original paper in German (l~lber den Einfluss der Schichtarbeit den Gesundheitszustand und das Kraakheitsgeschehen der Werkt~itigen). From the Proceedings of an International Symposium, January 31-February 1. 1969. Oslo, Norway, pp. 124-153. Translated by Information Company of America, 2204 Walnut Street, Philadelphia, PA 19103, 25 pages. Eastman Kodak Company, 1983. Shift work-use and effects. In: Ergonomic Design for People at Work, Vol. 2. Van Nostrand Reinhold, New York, pp. 288-303. Johnson L.C., Tepas, D.I., Colquhoun, W.P. and Colligan, M.J. (Eds.), 1981. The Twenty-Four Hour Workday: Pro-
ceedings of a Symposium on Variations in Work-sleep Schedules. DHHS (NIOSH) Publication No. 81-127. US Government Printing Office, Washington, DC. Monk, T.H. and Embrey, D.E., 1981. A field study of circadian rhythms in actual and interpolated task performance. In: A. Reinberg, N. Vieux. and P. Andlauer (Eds.), Night and Shift Work: Biological and Social Aspects. Oxford: Pergamon. Rosa, R.R. and Colligan, M.J., 1989. Extended workday: Effect of 8-hour and 12-hour rotating shift schedules on performance, subjective alertness, sleep patterns, and psychosocial variables. Work and Stress, 3 (1): 21-32. Rutenfranz, J., Colquhoun, W.P., Knauth, P. and Ghata, J.N., 1977. Biomedical and psychosocial aspects of shift work; A review. Scandinavian Journal of Work, Environment, and Health, 3: 165-181. Sergean, R., 1971. Managing Shiftwork. London: Gower Press, Industrial Society. Smith, M.J., 1987. Occupational Stress. In: Gavriel Salvendy (ed.), Handbook of Human Factors. New York: Wiley Interscience. Tepas, D.I., Armstrong, D.R., Carlson, M.L., Duchon, J.C., Gersten, A. and Lezotte, D.V., 1985. Changing industry to continuous operations: Different strokes for different plants. Behavior Research Methods, Instruments, and Computers, 17: 670-676. Vernon, H.M., 1920. The speed of adaptation of output to altered hours of work. Reports of the Industrial Fatigue Research Board No. 6. London: His Majesty's Stationery Office. Wever, R.A., 1981. On varying work-sleep schedules: The biological rhythm perspective. In: L.C. Johnson, D.I. Tepas, W.P. Colquhoun and M.J. Colligan (Eds.). The TwentyFour Hour Workday: Proceedings of a Symposium on Variations in Work-Sleep Schedules. DHHS (NIOSH) Publication No. 81-127. US Government Printing Office, Washington, DC, pp. 51-86. Wyatt, S. and Marriott, R., 1953. Night work and shift changes. British Journal of Industrial Medicine, 10: 164-168.
63
Case study
Rotating-shift system vs. fixed-shift system T i a n - S h y L i o u a n d M a o - J i u n J. W a n g Department of lndustrial Engineering, National Tsing Hua University, Hsin-Chu, Taiwan, ROC
(Received April 4, 1990; accepted in revised form June 4, 1990)
Abstract The performances of two different shift work systems were assessed in a resistor manufacturing company over a period of 13 months. At first, the existing rotating-shift system was evaluated; the yield and productivity were found to be significantly affected by shift and working day factors ( p < 0.05). The day shift had the best performance followed by swing and night shift. Both the yield and productivity rose for the first four working days, then declined for the following two days. Subsequently, a new fixed-shift system was proposed and the performances again were evaluated. Once again, the shift effect was found to be significant for both productivity and yield. Comparing the differences between the two shift systems, both productivity and yield have shown significant differences in swing and day shift (p <0.05). For the night shift, performance difference between the rotating-shift and the fixed-shift systems was found at p < 0.1. In summary, the fixed-shift system was found to be superior to the rotating-shift system in many aspects. A few months after implementing the fixed-shift system, the company decid-.d to return to the rotating-shift system due to the high turnovers involved in the night-shift workers. The work shift system design implication is also discussed.
Relevance to industry This paper presents the finding of evaluating two work-shift systems in a resistor manufacturing company. Since this is a real case study, it is obvious that the work is directly relevant to industry. Keywords Rotating shift, fixed shift, yield, productivity, circadian rhythms, sleep deprivation.
1. Introduction Shift w o r k is d e f i n e d as w o r k i n g o t h e r t h a n d a y t i m e hours. T h e system of three-shift r o t a t i o n s h a s been d e v e l o p e d since the 1920s (Sergean, 1971). T h e early reasons for e x t e n d i n g h o u r s b e y o n d the d a y t i m e shift were b a s e d o n a need for continuous services o r c o n t i n u o u s o p e r a t i o n s , for exa m p l e , g e n e r a t i n g electrical utilities o r c h e m i c a l m a n u f a c t u r i n g processes. In recent years, due to r a p i d changes in t e c h n o l o g y a n d increased g l o b a l c o m p e t i t i o n in the m a r k e t place, the g r o w i n g a p p l i c a t i o n s o f a shift w o r k system have b e e n l i n k e d to the e c o n o m i c s o f m a n u f a c t u r i n g . By using the e q u i p m e n t a r o u n d the clock, the c o m p a n y can 0169-1936/91/$03.50 © 1991 - Elsevier Science Publishers B.V.
recover the c a p i t a l i n v e s t m e n t m u c h earlier t h a n the p r o d u c t ' s life cycle ( E a s t m a n K o d a k , 1983). However, research has i n d i c a t e d that shift work, a n d night w o r k in p a r t i c u l a r , m a y have a n a d v e r s e effect on the h e a l t h a n d w e l l - b e i n g of the w o r k e r s ( J o h n s o n et al., 1981; R u t e n f r a n z et al., 1977). This is a t t r i b u t e d to the fact t h a t w o r k e r s have to w o r k at times which are d i s r u p t i v e to their p h y s i o logical, p s y c h o l o g i c a l a n d social c i r c a d i a n r h y t h m s (Aschoff, 1981; Wever, 1981). T h e effect is m a n i fested not o n l y in d a i l y life b e h a v i o r b u t also in j o b p e r f o r m a n c e . U s u a l l y the j o b p e r f o r m a n c e s e v a l u a t e d in shift w o r k studies were p r o d u c t i v i t y , e r r o r rate a n d a c c i d e n t r a t e ( E a s t m a n K o d a k , 1983). T h e shift effect o n a c c i d e n t rate has b e e n s t u d i e d b y various investigators, b u t n o c o n s i s t e n t
Tian-Shy Liou, Mao-Jiun J. Wang / Rotating- t's. fixed-shift system
64
d Start -I the capping machine
findings have been reported (Vernon, 1920; Wyatt and Marriott, 1953; Brandt, 1969). In addition, performance evaluation based on accident rate tends to be less representative when the observation period is not long enough. Many types of shift work systems are available. The selection of a suitable shift system has to consider the production needs, the job requirements (physical or mental), and the psychological consequences of the schedule. General guidelines for the design and evaluation of shift schedules are available in various human factors references (e.g. Eastman Kodak, 1983; Smith, 1987). In this study, the performance of a rotating-shift work system in a resistor manufacturing company has been evaluated. Subsequently, a new fixed-shift work system was proposed and the performance measures in terms of productivity and yield were further evaluated.
L I vibration AO,and s,,o J L IMake sure capping math speed
r
ake appropriate~ actions |
~-ine operate smooth y
I theOe,erm,oe I I causes
,osoect ]
the capped Ros
and check quantity
2. Problem situation
The study was conducted in an electronics company which produces resistors. There were 180 employees. Five major operations are involved in the resistors manufacturing process: roding, capping, helixing, cameling, and packaging automatically by a capping machine. The Ro is a ceramic rod after electroless nickel plating or magnetron sputtering. The operator's main task is to feed caps and Ros into the capping machine, to monitor the process, and to inspect the capped Ros. Each capped Ro consists of one Ro and two caps. The capped Ro is considered to be defective if only one cap or no cap was assembled. The description of the capping operation is shown in figure 1. The operation was originally conducted under a rotating-shift system which is a rotating hours and continuous work week system. Each
I subsequent Oet"Ooperation 'o I Fig. t. Task description of the capping operation.
operator worked six consecutive days, and then took two days off. A total of 24 operators was involved in the capping operation, and they were all males. The mean age was 29 years, ranging from 24 to 37 years. The average on the job experience was 4.2 years (range from 1.2 to 13 years). The 24 operators were divided into four groups with six workers in each group. The shifts rotated in a sequence of day, swing, and night. The three shift schedules were 07.30 to 16.00 h for day, 15.30 to
Table 1 Work schedule for a rotating-shift cycle (24 days complete a shift cycle). Day Group Group Group Group
1 2 3 4
M
T
W
Th
F
S
Su
M
T
W
Th
F
S
Su
M
T
W
Th
F
S
Su
M
T
W
D R S N
D R S N
D S R N
D S R N
D S N R
D S N R
R S N D
R S N D
S R N D
S R N D
S N R D
S N R D
S N D R
S N D R
R N D S
R N D S
N R D S
N R D S
N D R S
N D R S
N D S R
N D S R
R D S N
R D S N
D = day: S = swing: N = night; R = rest.
Tian-Shy Liou, Mao-Jiun J. Wang / Rotating- cs. fixed-shift system 24.00 h for swing, a n d 23.30 to 08.00 h for night. O n e r o t a t i n g cycle takes 24 d a y s a n d the schedule is d e m o n s t r a t e d in table 1. Each o p e r a t o r h a n d l e d two c a p p i n g m a c h i n e s s i m u l t a n e o u s l y ; the machine were not always a t t e n d e d b y the s a m e o p e r ator. A total of 12 c a p p i n g m a c h i n e s is in o p e r a t i o n d u r i n g each shift.
3. The rotating-shift system 3.1. Performance evaluation o f the rotating-shift system F o r the r o t a t i n g - s h i f t system, p e r f o r m a n c e s were collected for a p e r i o d o f 192 d a y s which c o v e r e d eight shift cycles. A n e x p e r i m e n t analysis was a p p l i e d to e v a l u a t e factors affecting j o b performances. T h e i n d e p e n d e n t variables were w o r k ing g r o u p (4 groups), shift (3 levels: day, swing, a n d night), w o r k i n g d a y (6 days), a n d o p e r a t o r (6 people). T h e eight shift cycles were c o n s i d e r e d as eight replications in each c o n d i t i o n . T h e perform a n c e measures were yield a n d p r o d u c t i v i t y . T h e y i e l d was defined as the ratio b e t w e e n o u t p u t a n d i n p u t in percentage. T h e i n p u t a n d o u t p u t were m e a s u r e d b y a specific electronic b a l a n c e with ' p i e c e s ' as its unit. T h e p r o d u c t i v i t y (with n u m b e r of pieces p e r h o u r as its unit) was d e f i n e d as the r a t i o of o u t p u t to the actual o p e r a t i o n time, which excludes a m a c h i n e ' s d o w n time, a n d idle time d u e to insufficient m a t e r i a l supplies. T h e i n p u t a n d o u t p u t were r e c o r d e d b y the o p e r a t o r s a n d c h e c k e d b y the s t o c k m a n . D a i l y p r o d u c t i v i t y a n d yield were p o s t e d on the next day. T h e p e r f o r m a n c e d a t a were a n a l y z e d b y SAS, which is a statistical analysis software. 3.2. Results and discussion T w o f o u r - w a y analyses o f variances were perf o r m e d on yield a n d p r o d u c t i v i t y respectively. T h e results are p r e s e n t e d in tables 2 a n d 3. F o r the m a i n effects, the shift a n d w o r k i n g d a y factors have d e m o n s t r a t e d significant influences (p < 0.001) on b o t h yield a n d p r o d u c t i v i t y . But the w o r k i n g g r o u p a n d o p e r a t o r factors were not significant. T h e lack o f i n d i v i d u a l differences is d u e to the fact that the o p e r a t i o n was p a c e d b y the machine. All of t h e m were e x p e r i e n c e d o p e r a t o r s
65
Table 2 Analysis of variance for yield under the rotating-shift system. Factor
df
Mean square
F value
Group (G) Shift (S) Day (D) Operator (O) G×S G×D G×O Sx D Sx O Dx O G x S× D GxSxO GxDxO Sx D x O GxSxDxO Error
3 2 5 5 6 15 15 10 10 25 30 30 75 50 150 3024
0.07 633.10 91.47 0.03
1.64 13102.11 a 1839.16 a 0.69 0.05 0.04 0.85 32.52 ~ 0.04 0.03 0.05 0.04 0.03 0.04 0.04
0.04 1.57
0.04
Note: The data is truncated to the second digit after the decimal point, a: Significant at p < 0.001; - : < 0.01.
w h o h a d fairly close skill levels, a n d were p a i d b y m o n t h l y salary. R e g a r d i n g i n t e r a c t i o n effects, o n l y the shift × w o r k i n g d a y factor was significant (p < 0.001) for b o t h p r o d u c t i v i t y a n d yield. T h e rest of the i n t e r a c t i o n s were not significant. T h e significant t w o - w a y i n t e r a c t i o n (shift × w o r k i n g d a y ) for b o t h yield a n d p r o d u c t i v i t y is illustrated in figures 2 a n d 3 respectively. W e see that the d a y
Table 3 Analysis of variance for productivity under the rotating-shift system. Factor
df
Mean square
F value
Group (G) Shift (S) Day (D) Operator (O) G×S G×D GxO Sx D Sx O DxO G x Sx D G x Sx O Gx Dx O Sx D x O G×S×D×O Error
3 2 5 5 6 15 15 10 10 25 30 30 75 50 150 3024
39.02 814714.08 116635.20 6.18 1.69 0.84 25.60 1857.53 1.62 1.59 1.85 1.80 1.72 2.45 1.87 53.03
0.74 15363.09 a 2199.39 a 0.12 0.03 0.02 0.48 35.03" 0.03 0.03 0.03 0.03 0.03 0.05 0.04
": Significant at p < 0.001.
Tian-Shy 1.iou, MaoJiun J. Wang / Rotating- vs. fixed-shift system
66 99
98
o
,--I
97
z<
96
95
I
I
I
1
2
3
'"
I
I
4
5
""
I
6
WORKING DAY Fig. 2. Mean yield for the three shifts across six working days under the rotating-shift system.
shift had the best performance followed by swing and night shifts for both yield and productivity. One immediate difference is that more supervisors and managers were around in the day-shift than in the night-shift. In addition, the major contributing factor is due to variation in the effect of rhythmic changes in physiological functions during a 24hour period, with performances dropping during the night, rising during the day, and reaching a peak in the afternoon. The rhythmic changes in physiological functions have been found to be
associated with changes in performance (Monk and Embrey, 1981). Further, Tepas et al. (1985) investigated sleep lengths of the shift workers and reported that night-shift workers slept the least among the three shift workers. Thus performance of the night-shift workers may suffer not only from workers being at a low ebb of the circadian cycle, but also from sleep deprivation effect. Further, performance was found to reach a peak on the fourth working day, declining again on the fifth and sixth days, with the first working day
8400
8200
8000 I'-"
O
A
SWING
7800
t-~
m =ooo,. 0.. z< uJ
.,~
7600
7400
7200
I
I
1
2
"""
I
I
I
I
3
4
5
6
WORKING DAY
Fig. 3. Mean productivity for the three shifts across six working days under the rotating-shift system.
67
Tian-Shy Liou. Mao-Jiun J. Wang / Rotating- vs. fixed-shift system
Table 5 Analysis of variances for yield and productivity under the fixed-shift system.
being the lowest. This is again attributed to the fact that the operator's circadian rhythm was unadjusted after two days off. The unadjustment tends to improve gradually and approaches a peak performance on the fourth working day of a week. It is possible that due to the cumulative fatigue effect the performances dropped during the last two days. Furthermore, a Pearson correlation was conducted between productivity and yield, and a significant positive correlation ( r - - 0 . 9 2 8 , p < 0.001) was found. This indicates that no trade-off effect was found between speed (productivity) and accuracy (yield) performances. The shift and working day factors had a significant influence on both speed and accuracy performances.
Factor
Shift Error
df
2 357
Yield Mean square 79.08 0.11
Productivity Mean F value square
F value 6.92.18a
137 442.03 185.59
740.56 a
Note: The data is truncated to the second digit after the decimal point, a: significant at p < 0.001.
ble for evaluation; 120 replications were involved in each shift level. The dependent variables were again the productivity and yield. 4.2. Result and discussion
4. The fixed-shift system The one-way analyses of variances were performed, and the results are shown in table 5. The shift effect was again found to be significant ( p < 0.001) for both productivity and yield. The average performances in terms of productivity and yield for the three shifts are demonstrated in figures 4 and 5. As expected, the day-shift had the best performance, followed by swing-shift and night-shift. In addition to the previously mentioned reasons of circadian rhythmic adjustments and sleep deprivations, Rosa and Colligan (1989) also reported that the self-rating stress of the day workers was perceived to be the highest a m o n g the three groups, and that of the night workers was the lowest. The workers in this study also indicated that they paid more attention to the j o b in the day-shift than in the night-shift due to the fact that more supervisors and managers were around in the day-shift than in the night-shift. Perhaps the stress due to a close supervision was also a contributing factor to the increased performances of the day-shift workers. Furthermore, the result of
4.1. Performance evaluation of the fixed-shift system
After evaluating the rotating-shift system, a fixed-shift working system was subsequently proposed for evaluation. In the new system, the 24 shift workers were divided into three groups (i.e. day, swing, and night) with eight workers in each group. Similar to the rotating-shift system, each operator worked six consecutive days, and then took two days off. Also within each shift group, only six out of eight workers were on duty, and the remaining two workers were off. The detailed arrangement for the fixed-shift system is shown in table 4. Since the fixed-shift system was a newly adapted system, a two-month adjustment period was given to help the workers get used to the new working schedule. Afterwards, performances over 120 days (15 cycles) were collected. Due to the lack of significant effects on group and operator factors found in the rotating shift system, only the shift factor was utilized as the independent variaTable 4
Work and rest schedule for a fixed-shift cycle. The arabic numerals represent the workers (8 days complete a shift cycle). Day Rest operators Operator from work to rest Operator from rest to work
M 1,2
T 2,3
W 3,4
Th 4,5
F 5,6
S 6,7
Su 7,8
M 8,1
2
3
4
5
6
7
8
1
8
1
2
3
4
5
6
7
68
Tian-Shy Liou, Mao-Jiun J. Wang / Rotating- vs. fixed-shifi system 99 FIXED-SHIFT
q w
98
>Z
< ILl
~E 97 ¸
96
I
!
I
day
swing
night
WORK SHIFT
Fig. 4. Mean yield for each shift under the fixed-shift and rotating-shift system respectively. Pearson correlation between yield and productivity indicated that a significant positive correlation ( r = 0.851, p < 0.001) was found. This again reveals the non-existence of speed and accuracy trade-off effect. Both productivity and yield were significantly affected by the shift factor.
5. Comparison of the two systems In order to evaluate the performance differences between the rotating-shift and the fixed-
shift system, six paired t-tests were conducted. The results are summarized in table 6. Significant differences were found on productivity and yield ( p <0.05) between the rotating-shift and the fixed-shift system in both swing and day shifts. As for the night shift, significant performance differences were found at p < 0.10. Figures 4 and 5 demonstrate the mean performances of the three shifts under the two shift work systems. It is obvious that the fixed-shift system is superior to the rotating-shift system in both productivity and yield. The superiority of the fixed-shift system is
8600 FIXED-SHIFT
8400 A
_> .¢: (.) ca
8200
8I I " _ = ca
8000
7800
7600
Day
Swing
Night
WORKSHIFT Fig. 5. Mean productivity for each shift under the fixed shift and the rotating-shift system respectively.
Tian-Shy Liou, Mao-Jiun J. Wang / Rotating- vs. fixed-shift system
69
Table 6 Significant differences on mean yield and mean productivity between the rotating-shift and the fixed-shift systems under day, swing, and night shift. Fixed-shift
Rotating-shift
t-value
Mean yield (%) Day
M
SD Swing
M
SD Night
M
SD
98.36 0.23 97.66 0.28 96.84 0.28
M
SD M
SD M
SD
97.25 0.49 96.95 0.48 96.24 0.49
2.37 ~
8136.90 148.45 7835.90 119.79 7611.60 151.45
2.72 "
1.86 b 1.48 ~"
Mean productivity ( p c s / h ) Day
M
SD Swing
M
SD Night
M
SD
8450.30 67.54 8127.50 89.20 7800.40 103.27
M
SD M
SD M
SD
2.76 a 1.53 -
Note: df = 310; a: significant at p < 0.01; b: significant at p < 0.05; +: significant at p < 0.1.
not confounded with time because all the operators had at least one year on the job. Therefore, the possible involvement of learning effect is very slim. Further, it is interesting to note that all the standard deviations in the fixed-shift performances are smaller than that of the rotating-shift performances. This indicates the fixed-shift system had more consistent performances than the rotating-shift system. Overall, the fixed-shift system has demonstrated a significantly better performance than the rotating-shift system in many aspects. This may be attributed to the fact that workers who work under the fixed schedule have less circadian rhythm adjustment problems than the rotating-shift workers. Furthermore, it is interesting to find that the performance on the fourth working day of a week in the rotating-shift system is very close to the average performance of the fixed-shift system (table 6 and figures 2 and 3). In other words, the peak performance in the rotating-shift system is about the same as the average performance of the fixed-shift system. Based on these findings, it seems that it is logical for the company to continue implementing the fixed-shift system.
6. Concluding remarks In summary, through 13 months' evaluation, the result of this study indicated that performances in terms of productivity and yield of the fixed-shift schedule are superior to those of the rotating-shift schedule. From interviews with operators it appeared that most of them felt that the fixed-shift system was better, but most of them prefer not to work at night. By providing an appropriate incentive, an increase of about 30% on the day shift salary, the company was able to attract operators to work at night under the permanent hour schedule. On the other hand, it was suggested that after implementing the system the company should also systematically evaluate the additional incentive cost, and the possible detrimental effects due to sleep deprivation and rhythmic changes of the night workers. After a few months' observations, the company decided to return to the rotating-shift system, due to the high turnovers involved in the night shift workers. Even though the company has provided some incentives to the night workers, the company
70
Tian-Shy Liou. Mao-Jiun J. Wang / Rotating- vs. fixed-shift system
still f o u n d it d i f f i c u l t to keep t h e m a n d to recruit n e w w o r k e r s for t h e n i g h t shift o p e r a t i o n . O n e p o s s i b l e e x p l a n a t i o n is that the j o b was d o n e by m a l e o p e r a t o r s , w h o c a n easily find a j o b e l s e w h e r e w i t h o u t s u f f e r i n g d i s r u p t i o n s to their r e g u l a r social lives a n d p h y s i o l o g i c a l r h y t h m s .
Acknowledgments T h e a u t h o r s wish to a c k n o w l e d g e w i t h g r a t i t u d e the c o o p e r a t i o n a n d s u p p o r t o f the staffs, m a n a g e m e n t s a n d shift w o r k e r s at Y o n g t a r e s i s t o r m a n u f a c t u r i n g c o m p a n y at K a o h s u n g , T a i w a n .
References Aschoff. J.. 1981. Circadian rhythms: Interference with and dependence on work-rest schedules. In: L.C. Johnson, D.I. Tapas. W.P. Colquhoun and M.J. Colligan (Eds.), The Twenty-Four Hour Workday: Proceedings of a Symposium on Variation in Work-Sleep Schedules. DHHS (NIOSH) Publication No. 81-127. US Government Printing Office, Washington, DC, pp. 13-50. Brandt. A., 1969. Influence of shift work on the health and pathology of workers. Original paper in German (l~lber den Einfluss der Schichtarbeit den Gesundheitszustand und das Kraakheitsgeschehen der Werkt~itigen). From the Proceedings of an International Symposium, January 31-February 1. 1969. Oslo, Norway, pp. 124-153. Translated by Information Company of America, 2204 Walnut Street, Philadelphia, PA 19103, 25 pages. Eastman Kodak Company, 1983. Shift work-use and effects. In: Ergonomic Design for People at Work, Vol. 2. Van Nostrand Reinhold, New York, pp. 288-303. Johnson L.C., Tepas, D.I., Colquhoun, W.P. and Colligan, M.J. (Eds.), 1981. The Twenty-Four Hour Workday: Pro-
ceedings of a Symposium on Variations in Work-sleep Schedules. DHHS (NIOSH) Publication No. 81-127. US Government Printing Office, Washington, DC. Monk, T.H. and Embrey, D.E., 1981. A field study of circadian rhythms in actual and interpolated task performance. In: A. Reinberg, N. Vieux. and P. Andlauer (Eds.), Night and Shift Work: Biological and Social Aspects. Oxford: Pergamon. Rosa, R.R. and Colligan, M.J., 1989. Extended workday: Effect of 8-hour and 12-hour rotating shift schedules on performance, subjective alertness, sleep patterns, and psychosocial variables. Work and Stress, 3 (1): 21-32. Rutenfranz, J., Colquhoun, W.P., Knauth, P. and Ghata, J.N., 1977. Biomedical and psychosocial aspects of shift work; A review. Scandinavian Journal of Work, Environment, and Health, 3: 165-181. Sergean, R., 1971. Managing Shiftwork. London: Gower Press, Industrial Society. Smith, M.J., 1987. Occupational Stress. In: Gavriel Salvendy (ed.), Handbook of Human Factors. New York: Wiley Interscience. Tepas, D.I., Armstrong, D.R., Carlson, M.L., Duchon, J.C., Gersten, A. and Lezotte, D.V., 1985. Changing industry to continuous operations: Different strokes for different plants. Behavior Research Methods, Instruments, and Computers, 17: 670-676. Vernon, H.M., 1920. The speed of adaptation of output to altered hours of work. Reports of the Industrial Fatigue Research Board No. 6. London: His Majesty's Stationery Office. Wever, R.A., 1981. On varying work-sleep schedules: The biological rhythm perspective. In: L.C. Johnson, D.I. Tepas, W.P. Colquhoun and M.J. Colligan (Eds.). The TwentyFour Hour Workday: Proceedings of a Symposium on Variations in Work-Sleep Schedules. DHHS (NIOSH) Publication No. 81-127. US Government Printing Office, Washington, DC, pp. 51-86. Wyatt, S. and Marriott, R., 1953. Night work and shift changes. British Journal of Industrial Medicine, 10: 164-168.