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Manual assembly learning and performance of left- and right-handers
Industrial Ergonomics ELSEVIER
International Journal of Industrial Ergonomics 19 (1997) 41-47
Manual assembly learning and performance of left- and right-handers Errol Hoffmann *, Johnny Halliday Department of Mechanical and Manufacturing Engineering, Universi~ of Melbourne, Parkville. Victoria 3052. Australia
Received 27 February 1995; revised 10 October 1995
Abstract This study investigated the learning and performance ability of left-handers in comparison to right-handers in psychomotor tasks. Ten left-handers and 10 right-handers were observed assembling 24 hacksaws and 39 U-bolts and their learning and performance was measured. No significant difference between the performance and learning ability of left-handers in comparison to right-handers was found in either task. Analysis of variance showed that there was significant learning in the first two trials during the U-bolt assembly and in the first five trials during the Hacksaw assembly. The standard deviations for each trial again showed no significant differences between left- and right-handers. A significant decrease in the standard deviation was found in the first three trials of Hacksaw assembly.
Relevance to industry Left- and right-handed workers are commonly used in assembly tasks. Previous work has suggested that right-handers may be better in their learning of motor tasks. This research shows that, for real assembly work, there is no difference in performance of left- and fight-handed persons, thus no special selection criteria are needed. Keywords: Manual assembly; Handedness; Assembly times
1. Introduction Throughout history, handedness has been surrounded by social stigma and stereotype. The belief that left-handers are inferior to right-handers has been and still is evident in many spheres of life ranging from education through to religion (Hardyck and Petrinovich, 1977; Coren, 1992). In recent history many scientists have disputed this stereotype
* Corresponding author.
(Chapanis and Gropper, 1968; Coren, 1992; Schmauder et al., 1993; W o l f et al., 1977). Hoffmann (1996) found that there was no significant performance difference between left- and rightbanders in ballistic and visually controlled movement using their preferred hand. However, he found that in some tasks left-handers performed significantly better with their non-preferred hand than did right-handers. Schmauder et al. (1993) found that right-handers were less universally employable than left-handers in assembly work, and that left-handers have equivalent strength in both arms to the preferred hand of right-handers.
0169-8141/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSDI 0 169-8141 (95)00091-7
42
E. Hoffmann, J. Halliday / International Journal of h~dustrial Ergonomics 19 (1997) 41-47
Salvendy and Seymour (1973, pp. 301-302) refer to other work which found that experienced industrial workers who were left-handed performed as well as right-handed industrial workers. However, learning was not investigated as an already experienced group was used. One of the few studies comparing the learning and performance ability of left- and right-handers in psychomotor tasks is that of Salvendy (1970) who tested left- and right-handed subjects on five different apparatus; the One-hole test, the Purdue pegboard, the rotary pursuit apparatus, the Hand dynamometer and the blind positioning apparatus. Salvendy found no significant difference between leftand right-handers in learning ability using the Purdue pegboard with their preferred hand and no significant difference in hand strength between left- and righthanders. However, he found that left-handers had superior learning with the rotary pursuit apparatus. The likely reason for this difference was that the rotary pursuit revolved clockwise for both groups, thus not taking into account the handedness of the subjects. Overall, Salvendy (1970) concluded that the learning and performance ability of left-handers in psychomotor tasks was inferior to right-handers. These conclusions do not all appear to follow from his results, which indicate little overall difference between the two groups. Salvendy and Seymour (1973, pp. 229-230) note that the differences in learning, although statistically different, were practically insignificant. Salvendy (1970) suggested that turther investigation was necessary in order to support or negate his findings. The present study further investigates the learning and performance ability of left-handers in comparison to right-handers in psychomotor tasks. Thus the aims of the present research were to test the learning ability and performance of left-handers in comparison to right-handers in simple psychomotor tasks and to test the hypothesis that there is no significant difference between left- and right-handers in task performance.
signed; these involved manual assembly of hacksaws and U-bolts in simulations of real industrial tasks. The workplace layout and assembly tasks are described in the following sections. 2.1. E q u i p m e n t 2.1.1. H a c k s a w s
The parts for 24 hacksaws were laid out using an ergonomic design (Fig. 1). All components were within the zone of convenient reach and in locations that allowed maximum use of simultaneous motions. Two jigs were constructed; one for right-handers and one for left-handers. The two jigs were basically mirror images of one another but it was necessary to have the hacksaw constructed upside down on the right-handed jig so that the screw hole on the saw handle could be reached and so that the right hand could be used for turning the wing-nut. The experimenter demonstrated the desired method of assembly. For the left-handers this was: (i) reach for bow and handle simultaneously with the left and right hands (Fig. 1) and place these components on the jig, inserting the bow into the handle (ii) with the right hand reach for the screw and screw the bow to the handle, (iii) get and insert the bolt into the bow end, (iv) get blade and insert over the pins at each end of the frame, (v) get washer and place over bolt, (vi) get wing nut and screw onto bolt until blade is taut. 2.1.2. U-bolts
Thirty-nine, 60 mm U-bolts were constructed by each subject. A small jig held the U-bolt while it was being assembled. All six parts were placed using an
2. Method
In order to test the performance of left- and right-handers, two simple assembly tasks were de-
Fig. 1. Layout of the workplace for hacksaw assembly. The arrangement shown is for left-handers.The right-handedjig placed the hacksaw blade at the top.
E. Hoffmann, J. Halliday / International Journal of lndustrial Ergonomics 19 (1997) 41-47 These reversed for left-handers
Fig. 2. Layout of the workplace for U-bolt assembly. Arrangement shown is for right-handers.
ergonomic design which was mirrored for lefthanders (Fig. 2). The U-bolt assembly required simultaneous hand motion. The subjects were instructed to pick up the U-bolt (with the non-preferred hand) and the plate (with the preferred hand) together, the two washers together and the two bolts together. The required technique was demonstrated by the experimenter.
43
mance was to be recorded. The main instructions were to concentrate on the task and to work at a maximum speed. Each subject was seated in an upright position and their chair adjusted so that their elbows were 50-100 mm above the working surface of the table. They were not allowed practice trials before the start of each experiment as their learning was to be examined during the experimental trials. The subjects were timed constructing 39 U-bolts and 24 hacksaws. No rest periods were provided during the construction of the U-bolts or the hacksaws but a five-minute rest was provided between the completion of the U-bolt/hacksaw assembly and the beginning of the next assembly. The first group of tools to be assembled was alternated between subjects. Each assembly required the use of both hands but the assembly was designed so that the preferred hand was used more frequently and for the more difficult tasks.
2.2. Determination of handedness Each subject first filled out a questionnaire in order to determine the extent of their handedness (Coren, 1992). The questionnaire consisted of 12 questions such as "Which hand would you use to throw a ball most accurately?" Only strongly lateralised subjects were used, that is, those who responded with all-left or all-right responses.
2.3. Subjects Ten strongly lateralised right-handers (6 males and 4 females) and 10 strongly lateralised left-handers (4 males and 6 females) were used. Their ages ranged between 17 and 45 and they were of varying careers and backgrounds. No subjects had previous experience in assembly work. They all reported good health and had no physical disability.
2,4. Procedure Each subject was tested in the two assembly tasks; the construction of Hacksaws and U-bolts. The method of assembly for the Hacksaws and U-bolts was demonstrated twice and the subjects were instructed to use the exact method demonstrated. Subjects were informed that their learning and perfor-
3. Results
3.1. Hacksaw assembly The mean and standard deviations of assembly times for each trial are given in Table 1. Mean data are shown in Fig. 3. There were no errors made in the assemblies, thus only assembly times are discussed in the following.
3.1.1. Analysis of variance A three-factor, mixed model analysis of variance was carried out on individual times for each assembly (Table 1), the variables being handedness (left or right), trial number (24) and subjects (10). No significant difference was found between the learning and performance ability of left- and right-handers. Trial number was found to have a significant effect on the time taken (F(23,414)= 9.21, p < 0.01). Significant learning was found in the first five trials. Post-hoc tests, using the Newman-Keuls procedure, showed that the first trial was significantly different to all others ( p < 0.01), the second trial was significantly different to trial numbers 6 - 2 4 ( p < 0.01), and the third and fourth trials were significantly different to trial numbers 11-24 ( p <
E. Hoffmann, J. Halliday / International Journal of Industrial Ergonomics 19 (1997) 41-47
44
Table l Mean times and standard deviations for left- and right-handers performing a hacksaw assembly task Trial Mean assembly times number (sec) for each trial
Standard deviation (sec) for each trial
Right-handers Left-handers Right-handers Left-handers l 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
52.9 45.8 41.2 41.1 43.7 36.3 39.9 39.2 34.4 39.1 34.0 37.3 34.7 35.2 33.2 37.3 35.4 31.2 38.7 34.6 36.2 32.9 34.3 32.2
52.9 44.7 43.6 45.2 44.2 39.5 39.8 42.2 34.2 33.6 35.5 34.6 35.1 35.3 34.4 36.4 37.0 38.6 34.8 35.5 31.6 35,0 33.6 32.3
18.7 l 1.0 14.9 8.7 15.8 8.7 9.8 9.0 7.8 7.4 6.0 10.1 8.5 9.5 9.0 9.4 6.9 5.3 10.4 7.1 10.9 4.9 4.7 10.5
14.7 9.2 12.8 8.2 15.6 13.1 11.9 10.3 8.5 5.1 5. l 7.8 5.5 7.7 6.2 9.2 12.0 17.7 14.6 7.7 7.2 6.4 6.7 7.1
3.1.2. Regression analysis Regression of the mean data (Table l) was attempted in terms of the De Jong (1957) equation for a learning curve; T,, = T I M + ( l - M ) / n
m,
where T l is the time for the first trial, T, is the time for the nth trial, M is a 'factor of incompressibility' and m is the learning exponent. It was found that the best fit was provided by a straight power regression for both left- and right-handers, that is, with M = 0. The regression equations calculated for right-handers (RH) and left-handers (LH) were: RH:
T,, = 49.9/n°129; r 2 = 0.76,
LH:
;r =
51.42/n °.138, r 2 = 0.79.
Neither the coefficients or exponents of these regressions were significantly different. Fig. 3 shows the mean assembly times as a function of the trial number with these regression equations fitted for each group.
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0.05). The fifth trial was significantly different to trial numbers 9 - 2 4 ( p < 0 . 0 1 in most cases). In summary, the first five trials were found to be significantly different to the later trials. A two-factor analysis of variance was also performed on the standard deviations for each trial, the variables being handedness (left or right) and trial number. There was no significant difference between the standard deviations for left- and right-handers. Post-hoc tests showed that there was a significant decrease in the standard deviations over the first five trials. The first trial was significantly different to trial numbers 6 - 2 4 ( p < 0 . 0 1 ) , the fifth trial was significantly different to trial numbers 6 - 2 4 ( p < 0.01 in most cases) and the third trial was significantly different to trial numbers 9 - 2 4 ( p < 0.05). Fig. 4 shows the standard deviations for each trial plotted as a function of the trial number.
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E. Hoffmann, J. Halliday / lnternational Journal of Industrial Ergonomics 19 (1997) 41-47 20-
45
Table 2 Mean assembly times and standard deviations for left- and righthanders perfonrting U-bolt assembly Trial Mean assembly times number (sec) for each trial
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17.8 15.9 18.0 14.5 16.4 15.5 15.3 14.9 15.3 14.1 14.3 14.2 13.2 15.3 14.1 16.2 13.9 14.0 15.6 12.9 I7.9 13.5 14.3 13.3 14.3 13.2 13.0 14.4 14.8 13.8 14.8 14.3 13.4 14.9 13.3 15.2 14.0 13.1 15.9
5.9 7.0 2.5 2.9 3.4 2.1 2.3 3.2 2.2 2.9 4.6 2.0 2.8 3.5 3.6 2.1 2.1 1.8 4.4 2.5 4.1 2.1 5.1 6.9 3.4 2.3 3.2 2.0 3.0 3.3 3.2 5.3 4.5 3.0 3.0 2.0 5.1 2.6 2.6
5.3 3.6 4.5 2.0 3.4 4.0 2.5 2.2 3.0 2.2 2.7 3.6 2.5 3.2 2.1 5.6 2.4 1.9 5.1 1.7 3.7 4.1 2.9 1.7 4.2 2.8 2.9 3.3 4.4 1.8 6.1 4.9 2.9 3.5 2.0 2.9 4.6 2.1 4.7
5 ............................................
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Trial Number Fig. 5. Mean times for U-bolt assembly as a function of trial number, for left- and right-handers. Curves are for the fitted De Jong equations.
3.2.1. Analysis o f variance A three-factor mixed-model
analysis of variance
was carried out on individual times for each assem-
E. Hoffmann, J. Halliday / lnternational Journal of Industrial Ergonomics 19 (1997) 41-47
46
bly, the variables being handedness (left or right), trial number (39) and subjects (10). No overall difference was found between the learning and performance ability of left- and right-handers. Trial number was found to have a significant effect on time taken (F(38,684) = 2.592, p < 0.01). There was significant interaction ( F ( 1 4 , 2 5 2 ) = 2.1131, p < 0 . 0 5 ) between R H / L H and trials. In some individual trials, left-handers were found to be significantly better than right-handers and in others, right-handers were found to be superior. Post-hoc tests showed that the subjects had significant learning in the first two trials. The first trial was significantly slower than trial numbers 3 - 3 9 ( p < 0.01) and the second trial was significantly slower than trial numbers 8 - 3 9 ( p < 0.05). A two-factor analysis of variance was also performed on the standard deviations for each trial, the variables being handedness (left or right) and trial number. There was no significant difference between the standard deviations for left- and right-handers. There was also no significant difference between any
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of the trials. Fig. 6 shows the standard deviations for each trial as a function of the trial number.
3.2.2. Regression analysis It was found that the best regression equation to fit each data group was a straight power regression as found for hacksaw assembly. The regression equations were: RH:
Tn = 17.462/n °°65, r 2 = 0.33,
LH:
T,, = 17.12/n °°57, r 2 = 0.3.
Again, neither the coefficients or exponents of these regressions were significantly different. These fitted regression equations for each set of data are shown in Fig. 5.
4. Discussion
The data of both experiments clearly show that there was no difference in the learning and performance ability of left-handers in comparison to righthanders. These results support the studies on performance by others such as Berry et al. (1980) who found that handedness had no effect on assembly so long as it was done independently of other activities (he found that left-handers did not perform as well on simultaneous spatial-sequential tasks). They also support the study discussed by Salvendy and Seymour (1973, pp. 301-302) which found no difference between the performance of left- and righthanded industrial workers. They contradict Salvendy's early conclusions (1970) that his results 'tend to support the hypothesis that right-handed subjects have higher performances and learning scores with their preferred and non-preferred hand than do left-handers'. Analysis of variance showed that significant learning was experienced for both groups during the first five trials in the hacksaw experiment and the first two trials in the U-bolt experiment. Even though the subjects had no practical experience assembling the hacksaws or U-bolts, they learned fairly quickly. The longer, significant, learning period for the hack-
E. Hoffmann, J. Halliday / lnternational Journal of lndustrial Ergonomics 19 (1997) 41-47
saws than the U-bolts is supported by the hacksaw's more complicated assembly and greater number of parts. Regression analysis showed that a power-law regression gave the best fit to the data in both experiments. A more complex and extended assembly process could be designed in order to investigate the learning process in more detail. In this study, the significant learning period was quite short and analysis was therefore limited. The tasks were also restricted to those in which close visual control was required because of the small tolerances between components. There may be cases where such close control is not necessary, but here it would be expected that, again, there would be no difference in performance of leftand right-handed persons. Analysis of variance showed a significant decrease in the standard deviation of trial times during the first five trials in the hacksaw experiment ( p < 0.05 in most cases). However, no change in the standard deviation of trial times was found in the U-bolt experiment. There was no significant difference between the standard deviations of trial times between left- and right-handers in both experiments.
5. Conclusions This study confirms what industry had already hypothesised, that left-handers are as productive in assembly work as right-handers. It supports the study by Salvendy and Seymour (1973) who found that
47
left-handed women work as well in assembly processes as do right-handers. It clearly shows that, contrary to earlier work by Salvendy, there is no difference in the rates of learning of left- and righthanders.
References Berry, G.A., Hughes, R.L. and Jackson, L.D., 1980. Sex and handedness in simple and integrated task performance. Perceptual and Motor Skills, 51: 807-812. Chapanis, A. and Gropper, B.A., 1968. The effect of the operator's handedness on some directional stereotypes in control-display relationships. Human Factors, 10: 303-320, Coren, S., 1992. The Left-hander Syndrome: The Causes and Consequences of Left-handedness. Free Press, New York. De Jong, J.R., 1957. The effects of increasing skill on cycle time and its consequences for time standards. Ergonomics, 2: 153166. Hardyck, C. and Petrinovich, L.F., 1977. Left-handedness. Psychological Bulletin, 84(3): 385-399. Hoffmann, E.R., 1996. Movement times of right- and left-handers using preferred and non-preferred hands. International Journal of Industrial Ergonomics, 19: 49-57. Salvendy, G., 1970. Handedness and Psychomotor Performance. Transactions, American Institute of Industrial Engineers, 2(3): 227-232. Salvendy, G. and Seymour, W.D., 1973. Prediction and Development of Industrial Work Performance. Wiley, New York. Schmauder, M., Eckert, R. and Schindhelm, R., 1993. Forces in the hand-ann system: Investigations of the problem of lefthandedness. International Journal of Industrial Ergonomics, 12: 231-237. Wolf, P.H., Huritz, I. and Moss, H., 1977. Serial organization of motor skills in left- and right-handed adults. Neuropsychologia, 15: 539-546.
International Journal of Industrial Ergonomics 19 (1997) 41-47
Manual assembly learning and performance of left- and right-handers Errol Hoffmann *, Johnny Halliday Department of Mechanical and Manufacturing Engineering, Universi~ of Melbourne, Parkville. Victoria 3052. Australia
Received 27 February 1995; revised 10 October 1995
Abstract This study investigated the learning and performance ability of left-handers in comparison to right-handers in psychomotor tasks. Ten left-handers and 10 right-handers were observed assembling 24 hacksaws and 39 U-bolts and their learning and performance was measured. No significant difference between the performance and learning ability of left-handers in comparison to right-handers was found in either task. Analysis of variance showed that there was significant learning in the first two trials during the U-bolt assembly and in the first five trials during the Hacksaw assembly. The standard deviations for each trial again showed no significant differences between left- and right-handers. A significant decrease in the standard deviation was found in the first three trials of Hacksaw assembly.
Relevance to industry Left- and right-handed workers are commonly used in assembly tasks. Previous work has suggested that right-handers may be better in their learning of motor tasks. This research shows that, for real assembly work, there is no difference in performance of left- and fight-handed persons, thus no special selection criteria are needed. Keywords: Manual assembly; Handedness; Assembly times
1. Introduction Throughout history, handedness has been surrounded by social stigma and stereotype. The belief that left-handers are inferior to right-handers has been and still is evident in many spheres of life ranging from education through to religion (Hardyck and Petrinovich, 1977; Coren, 1992). In recent history many scientists have disputed this stereotype
* Corresponding author.
(Chapanis and Gropper, 1968; Coren, 1992; Schmauder et al., 1993; W o l f et al., 1977). Hoffmann (1996) found that there was no significant performance difference between left- and rightbanders in ballistic and visually controlled movement using their preferred hand. However, he found that in some tasks left-handers performed significantly better with their non-preferred hand than did right-handers. Schmauder et al. (1993) found that right-handers were less universally employable than left-handers in assembly work, and that left-handers have equivalent strength in both arms to the preferred hand of right-handers.
0169-8141/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSDI 0 169-8141 (95)00091-7
42
E. Hoffmann, J. Halliday / International Journal of h~dustrial Ergonomics 19 (1997) 41-47
Salvendy and Seymour (1973, pp. 301-302) refer to other work which found that experienced industrial workers who were left-handed performed as well as right-handed industrial workers. However, learning was not investigated as an already experienced group was used. One of the few studies comparing the learning and performance ability of left- and right-handers in psychomotor tasks is that of Salvendy (1970) who tested left- and right-handed subjects on five different apparatus; the One-hole test, the Purdue pegboard, the rotary pursuit apparatus, the Hand dynamometer and the blind positioning apparatus. Salvendy found no significant difference between leftand right-handers in learning ability using the Purdue pegboard with their preferred hand and no significant difference in hand strength between left- and righthanders. However, he found that left-handers had superior learning with the rotary pursuit apparatus. The likely reason for this difference was that the rotary pursuit revolved clockwise for both groups, thus not taking into account the handedness of the subjects. Overall, Salvendy (1970) concluded that the learning and performance ability of left-handers in psychomotor tasks was inferior to right-handers. These conclusions do not all appear to follow from his results, which indicate little overall difference between the two groups. Salvendy and Seymour (1973, pp. 229-230) note that the differences in learning, although statistically different, were practically insignificant. Salvendy (1970) suggested that turther investigation was necessary in order to support or negate his findings. The present study further investigates the learning and performance ability of left-handers in comparison to right-handers in psychomotor tasks. Thus the aims of the present research were to test the learning ability and performance of left-handers in comparison to right-handers in simple psychomotor tasks and to test the hypothesis that there is no significant difference between left- and right-handers in task performance.
signed; these involved manual assembly of hacksaws and U-bolts in simulations of real industrial tasks. The workplace layout and assembly tasks are described in the following sections. 2.1. E q u i p m e n t 2.1.1. H a c k s a w s
The parts for 24 hacksaws were laid out using an ergonomic design (Fig. 1). All components were within the zone of convenient reach and in locations that allowed maximum use of simultaneous motions. Two jigs were constructed; one for right-handers and one for left-handers. The two jigs were basically mirror images of one another but it was necessary to have the hacksaw constructed upside down on the right-handed jig so that the screw hole on the saw handle could be reached and so that the right hand could be used for turning the wing-nut. The experimenter demonstrated the desired method of assembly. For the left-handers this was: (i) reach for bow and handle simultaneously with the left and right hands (Fig. 1) and place these components on the jig, inserting the bow into the handle (ii) with the right hand reach for the screw and screw the bow to the handle, (iii) get and insert the bolt into the bow end, (iv) get blade and insert over the pins at each end of the frame, (v) get washer and place over bolt, (vi) get wing nut and screw onto bolt until blade is taut. 2.1.2. U-bolts
Thirty-nine, 60 mm U-bolts were constructed by each subject. A small jig held the U-bolt while it was being assembled. All six parts were placed using an
2. Method
In order to test the performance of left- and right-handers, two simple assembly tasks were de-
Fig. 1. Layout of the workplace for hacksaw assembly. The arrangement shown is for left-handers.The right-handedjig placed the hacksaw blade at the top.
E. Hoffmann, J. Halliday / International Journal of lndustrial Ergonomics 19 (1997) 41-47 These reversed for left-handers
Fig. 2. Layout of the workplace for U-bolt assembly. Arrangement shown is for right-handers.
ergonomic design which was mirrored for lefthanders (Fig. 2). The U-bolt assembly required simultaneous hand motion. The subjects were instructed to pick up the U-bolt (with the non-preferred hand) and the plate (with the preferred hand) together, the two washers together and the two bolts together. The required technique was demonstrated by the experimenter.
43
mance was to be recorded. The main instructions were to concentrate on the task and to work at a maximum speed. Each subject was seated in an upright position and their chair adjusted so that their elbows were 50-100 mm above the working surface of the table. They were not allowed practice trials before the start of each experiment as their learning was to be examined during the experimental trials. The subjects were timed constructing 39 U-bolts and 24 hacksaws. No rest periods were provided during the construction of the U-bolts or the hacksaws but a five-minute rest was provided between the completion of the U-bolt/hacksaw assembly and the beginning of the next assembly. The first group of tools to be assembled was alternated between subjects. Each assembly required the use of both hands but the assembly was designed so that the preferred hand was used more frequently and for the more difficult tasks.
2.2. Determination of handedness Each subject first filled out a questionnaire in order to determine the extent of their handedness (Coren, 1992). The questionnaire consisted of 12 questions such as "Which hand would you use to throw a ball most accurately?" Only strongly lateralised subjects were used, that is, those who responded with all-left or all-right responses.
2.3. Subjects Ten strongly lateralised right-handers (6 males and 4 females) and 10 strongly lateralised left-handers (4 males and 6 females) were used. Their ages ranged between 17 and 45 and they were of varying careers and backgrounds. No subjects had previous experience in assembly work. They all reported good health and had no physical disability.
2,4. Procedure Each subject was tested in the two assembly tasks; the construction of Hacksaws and U-bolts. The method of assembly for the Hacksaws and U-bolts was demonstrated twice and the subjects were instructed to use the exact method demonstrated. Subjects were informed that their learning and perfor-
3. Results
3.1. Hacksaw assembly The mean and standard deviations of assembly times for each trial are given in Table 1. Mean data are shown in Fig. 3. There were no errors made in the assemblies, thus only assembly times are discussed in the following.
3.1.1. Analysis of variance A three-factor, mixed model analysis of variance was carried out on individual times for each assembly (Table 1), the variables being handedness (left or right), trial number (24) and subjects (10). No significant difference was found between the learning and performance ability of left- and right-handers. Trial number was found to have a significant effect on the time taken (F(23,414)= 9.21, p < 0.01). Significant learning was found in the first five trials. Post-hoc tests, using the Newman-Keuls procedure, showed that the first trial was significantly different to all others ( p < 0.01), the second trial was significantly different to trial numbers 6 - 2 4 ( p < 0.01), and the third and fourth trials were significantly different to trial numbers 11-24 ( p <
E. Hoffmann, J. Halliday / International Journal of Industrial Ergonomics 19 (1997) 41-47
44
Table l Mean times and standard deviations for left- and right-handers performing a hacksaw assembly task Trial Mean assembly times number (sec) for each trial
Standard deviation (sec) for each trial
Right-handers Left-handers Right-handers Left-handers l 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
52.9 45.8 41.2 41.1 43.7 36.3 39.9 39.2 34.4 39.1 34.0 37.3 34.7 35.2 33.2 37.3 35.4 31.2 38.7 34.6 36.2 32.9 34.3 32.2
52.9 44.7 43.6 45.2 44.2 39.5 39.8 42.2 34.2 33.6 35.5 34.6 35.1 35.3 34.4 36.4 37.0 38.6 34.8 35.5 31.6 35,0 33.6 32.3
18.7 l 1.0 14.9 8.7 15.8 8.7 9.8 9.0 7.8 7.4 6.0 10.1 8.5 9.5 9.0 9.4 6.9 5.3 10.4 7.1 10.9 4.9 4.7 10.5
14.7 9.2 12.8 8.2 15.6 13.1 11.9 10.3 8.5 5.1 5. l 7.8 5.5 7.7 6.2 9.2 12.0 17.7 14.6 7.7 7.2 6.4 6.7 7.1
3.1.2. Regression analysis Regression of the mean data (Table l) was attempted in terms of the De Jong (1957) equation for a learning curve; T,, = T I M + ( l - M ) / n
m,
where T l is the time for the first trial, T, is the time for the nth trial, M is a 'factor of incompressibility' and m is the learning exponent. It was found that the best fit was provided by a straight power regression for both left- and right-handers, that is, with M = 0. The regression equations calculated for right-handers (RH) and left-handers (LH) were: RH:
T,, = 49.9/n°129; r 2 = 0.76,
LH:
;r =
51.42/n °.138, r 2 = 0.79.
Neither the coefficients or exponents of these regressions were significantly different. Fig. 3 shows the mean assembly times as a function of the trial number with these regression equations fitted for each group.
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0.05). The fifth trial was significantly different to trial numbers 9 - 2 4 ( p < 0 . 0 1 in most cases). In summary, the first five trials were found to be significantly different to the later trials. A two-factor analysis of variance was also performed on the standard deviations for each trial, the variables being handedness (left or right) and trial number. There was no significant difference between the standard deviations for left- and right-handers. Post-hoc tests showed that there was a significant decrease in the standard deviations over the first five trials. The first trial was significantly different to trial numbers 6 - 2 4 ( p < 0 . 0 1 ) , the fifth trial was significantly different to trial numbers 6 - 2 4 ( p < 0.01 in most cases) and the third trial was significantly different to trial numbers 9 - 2 4 ( p < 0.05). Fig. 4 shows the standard deviations for each trial plotted as a function of the trial number.
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E. Hoffmann, J. Halliday / lnternational Journal of Industrial Ergonomics 19 (1997) 41-47 20-
45
Table 2 Mean assembly times and standard deviations for left- and righthanders perfonrting U-bolt assembly Trial Mean assembly times number (sec) for each trial
4"~k " "•. . . . . . . . . . . . . . . . . . . . . .
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1 2 3 4 5 6 7 8 9 10 1l 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39
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17.8 15.9 18.0 14.5 16.4 15.5 15.3 14.9 15.3 14.1 14.3 14.2 13.2 15.3 14.1 16.2 13.9 14.0 15.6 12.9 I7.9 13.5 14.3 13.3 14.3 13.2 13.0 14.4 14.8 13.8 14.8 14.3 13.4 14.9 13.3 15.2 14.0 13.1 15.9
5.9 7.0 2.5 2.9 3.4 2.1 2.3 3.2 2.2 2.9 4.6 2.0 2.8 3.5 3.6 2.1 2.1 1.8 4.4 2.5 4.1 2.1 5.1 6.9 3.4 2.3 3.2 2.0 3.0 3.3 3.2 5.3 4.5 3.0 3.0 2.0 5.1 2.6 2.6
5.3 3.6 4.5 2.0 3.4 4.0 2.5 2.2 3.0 2.2 2.7 3.6 2.5 3.2 2.1 5.6 2.4 1.9 5.1 1.7 3.7 4.1 2.9 1.7 4.2 2.8 2.9 3.3 4.4 1.8 6.1 4.9 2.9 3.5 2.0 2.9 4.6 2.1 4.7
5 ............................................
3.2. U-bolt assembly I ~ - Right-handers 0
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a r e g i v e n in T a b l e 2 a n d p l o t t e d i n F i g . 5.
Trial Number Fig. 5. Mean times for U-bolt assembly as a function of trial number, for left- and right-handers. Curves are for the fitted De Jong equations.
3.2.1. Analysis o f variance A three-factor mixed-model
analysis of variance
was carried out on individual times for each assem-
E. Hoffmann, J. Halliday / lnternational Journal of Industrial Ergonomics 19 (1997) 41-47
46
bly, the variables being handedness (left or right), trial number (39) and subjects (10). No overall difference was found between the learning and performance ability of left- and right-handers. Trial number was found to have a significant effect on time taken (F(38,684) = 2.592, p < 0.01). There was significant interaction ( F ( 1 4 , 2 5 2 ) = 2.1131, p < 0 . 0 5 ) between R H / L H and trials. In some individual trials, left-handers were found to be significantly better than right-handers and in others, right-handers were found to be superior. Post-hoc tests showed that the subjects had significant learning in the first two trials. The first trial was significantly slower than trial numbers 3 - 3 9 ( p < 0.01) and the second trial was significantly slower than trial numbers 8 - 3 9 ( p < 0.05). A two-factor analysis of variance was also performed on the standard deviations for each trial, the variables being handedness (left or right) and trial number. There was no significant difference between the standard deviations for left- and right-handers. There was also no significant difference between any
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Fig. 6. Standard deviations of assembly times for U-bolts as a function of trial number.
of the trials. Fig. 6 shows the standard deviations for each trial as a function of the trial number.
3.2.2. Regression analysis It was found that the best regression equation to fit each data group was a straight power regression as found for hacksaw assembly. The regression equations were: RH:
Tn = 17.462/n °°65, r 2 = 0.33,
LH:
T,, = 17.12/n °°57, r 2 = 0.3.
Again, neither the coefficients or exponents of these regressions were significantly different. These fitted regression equations for each set of data are shown in Fig. 5.
4. Discussion
The data of both experiments clearly show that there was no difference in the learning and performance ability of left-handers in comparison to righthanders. These results support the studies on performance by others such as Berry et al. (1980) who found that handedness had no effect on assembly so long as it was done independently of other activities (he found that left-handers did not perform as well on simultaneous spatial-sequential tasks). They also support the study discussed by Salvendy and Seymour (1973, pp. 301-302) which found no difference between the performance of left- and righthanded industrial workers. They contradict Salvendy's early conclusions (1970) that his results 'tend to support the hypothesis that right-handed subjects have higher performances and learning scores with their preferred and non-preferred hand than do left-handers'. Analysis of variance showed that significant learning was experienced for both groups during the first five trials in the hacksaw experiment and the first two trials in the U-bolt experiment. Even though the subjects had no practical experience assembling the hacksaws or U-bolts, they learned fairly quickly. The longer, significant, learning period for the hack-
E. Hoffmann, J. Halliday / lnternational Journal of lndustrial Ergonomics 19 (1997) 41-47
saws than the U-bolts is supported by the hacksaw's more complicated assembly and greater number of parts. Regression analysis showed that a power-law regression gave the best fit to the data in both experiments. A more complex and extended assembly process could be designed in order to investigate the learning process in more detail. In this study, the significant learning period was quite short and analysis was therefore limited. The tasks were also restricted to those in which close visual control was required because of the small tolerances between components. There may be cases where such close control is not necessary, but here it would be expected that, again, there would be no difference in performance of leftand right-handed persons. Analysis of variance showed a significant decrease in the standard deviation of trial times during the first five trials in the hacksaw experiment ( p < 0.05 in most cases). However, no change in the standard deviation of trial times was found in the U-bolt experiment. There was no significant difference between the standard deviations of trial times between left- and right-handers in both experiments.
5. Conclusions This study confirms what industry had already hypothesised, that left-handers are as productive in assembly work as right-handers. It supports the study by Salvendy and Seymour (1973) who found that
47
left-handed women work as well in assembly processes as do right-handers. It clearly shows that, contrary to earlier work by Salvendy, there is no difference in the rates of learning of left- and righthanders.
References Berry, G.A., Hughes, R.L. and Jackson, L.D., 1980. Sex and handedness in simple and integrated task performance. Perceptual and Motor Skills, 51: 807-812. Chapanis, A. and Gropper, B.A., 1968. The effect of the operator's handedness on some directional stereotypes in control-display relationships. Human Factors, 10: 303-320, Coren, S., 1992. The Left-hander Syndrome: The Causes and Consequences of Left-handedness. Free Press, New York. De Jong, J.R., 1957. The effects of increasing skill on cycle time and its consequences for time standards. Ergonomics, 2: 153166. Hardyck, C. and Petrinovich, L.F., 1977. Left-handedness. Psychological Bulletin, 84(3): 385-399. Hoffmann, E.R., 1996. Movement times of right- and left-handers using preferred and non-preferred hands. International Journal of Industrial Ergonomics, 19: 49-57. Salvendy, G., 1970. Handedness and Psychomotor Performance. Transactions, American Institute of Industrial Engineers, 2(3): 227-232. Salvendy, G. and Seymour, W.D., 1973. Prediction and Development of Industrial Work Performance. Wiley, New York. Schmauder, M., Eckert, R. and Schindhelm, R., 1993. Forces in the hand-ann system: Investigations of the problem of lefthandedness. International Journal of Industrial Ergonomics, 12: 231-237. Wolf, P.H., Huritz, I. and Moss, H., 1977. Serial organization of motor skills in left- and right-handed adults. Neuropsychologia, 15: 539-546.