Can pilots time-share better than non-pilots?

Can pilots time-share better than non-pilots?

Applied Ergonomics 1986, 17.4, 284-290 Ergonomics in aviation Can pilots time-share better than non-pilots? P.S. Tsang ResearchAssociate,Aviation Re...

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Applied Ergonomics 1986, 17.4, 284-290

Ergonomics in aviation

Can pilots time-share better than non-pilots? P.S. Tsang ResearchAssociate,Aviation ResearchLaboratory, University of Illinois at Urbana-Champaign, Savoy, II 61874, USA

Time-sharing performance of a group of pilots was compared with that of a group of college students. In a secondary task paradigm, both groups were required to perform five dual tasks with various degrees of structural similarity. A higher degree of task interference was observed for the structurally more similar task pairs. The data were consistent with the results from previous research and support the concept of multiple resources. Although the pilots appeared to be more efficient in one of the dual task conditions, evidence for a general difference in time-sharing ability between the students and the pilots was not compelling. It was concluded that the degree to which time-sharing performance is structure dependent is not easily alterable by training. The results suggested that laboratory findings on the structural determinants of time-sharing efficiency are generalisable to operational environments. Keywords: Flying operations, pilots, time-sharing

Introduction Multiple resource theory posits that there are multiple sources of attentional capacities each of which is somewhat dedicated to a specific type of information processing. For example, in the structure-specific resource model proposed by Wickens (1980), processing resources are defined by: (a) the stages of processing (perceptual/central vs response), (b) the codes of processing (spatial vs verbal), and (c) the input/output(I/O) modalities (visual vs auditory/manual vs speech). This model predicts that greater time-sharing efficiency can be achieved when the time-shared tasks place heavy demands upon different resources than when they are structurally similar and have to compete for the same

resources. Although these structural effects on time-sharing performance have been reported by several independent investigators, most of the studies were conducted with a rather specific sample of the population; namely, college students (e g, Gopher et al, 1982; McLeod, 1977; Tsang and Wickens, 1984; Wickens et al, 1981). Consequently, these results are often met with scepticism from practitioners and researchers alike. The generalisability of these effects is thus an important issue to address. The motivation for examining the generalisability question is twofold. First, it would give the practitioners a better idea of the extent to which they can apply laboratory results obtained with college sophomores to the experienced professionals in an operational environment. Second, it would lead to a better understanding of the structure of attentional resources. Implicit in the structure-specific resource model's predictions is the belief that time-sharing performance is, at

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least in part, structurally determined, and is universally so for all individuals. If so, training may alter the level of timesharing performance on the whole, but the relative amount of interference due to the structural properties of the timeshared tasks should be fairly resistant to practice, as long as the tasks remain resource-limited (Norman and Bobrow, I975). For example, at any given level of practice, there would be more interference between two manual response tasks than between a manual and a speech response task. In an experiment designed to identify a general time-sharing ability, Dames and Wickens (1980) obtained results which suggested that the extent of structural interference might be decreased by the development of time-sharing skills. The purpose of the present study is to examine the structural effects on time-sharing performance as a function of training. Specifically, the time-sharing performance of a group of college students was compared with that of a group of general aviation pilots. Time-sharing performance was examined as a function of: (1) the degree of structural similarity between the time-shared tasks, (2) the dynamics of the difficulty changes, and (3) the predictability of the difficulty changes. The premise of tl~-present paper is that pilots are highly practised in performing simultaneous activities in the cockpit. They are expected to have more experience dealing with time-varying task demands (as in maintaining the stability of an aircraft under severe wind gust conditions), and have had at least some training in handling unforeseeable changes in workload (such asa sudden increase in task demand due to an emergency). In the search for reliable predictive indicators for successful flight training, the relationship between timesharing performance and flight performance has been

0003-6870/86/04 0284-07 $03.00 © 1986 Butterworth & Co (Publishers) Ltd

Ergonomics in aviation examined in several studies. For example, Crosby and Parkinson (1979) had a group of experienced instructor pilots and a group of student pilots perform a ground controlled approach (GCA) as the primary task and a memory search task as the secondary task. The students' secondary task performance decrement was found to be significantly larger than that of the pilots. However, the superiority of the pilots' time-sharing ability remained questionable due to the disparate level of the GCA performance at the single task level, which must in some way affect the level of dual task performance (Ackerman et al, 1984). In another example, North and Gopher (1976) also found superior time-sharing performance in a group of flight instructors compared with a group of flight students. However, the comparison was based on only one pair of tasks (a one-dimensional tracking task and a discrete digit processing task) and the relative structural interferences of other task pairs were not examined. In the present study, a secondary task paradigm (see, for example, Ogden et al, 1979) was employed to examine five pairs of tasks with various degrees of structural similarity. All of the dual tasks had a compensatory tracking task as the primary task. As the secondary task, one pair (dual tracking) had another compensatory tracking task, and the other four pairs (transformation-tracking) had a spatial transformation task employing all possible combinations of two input (visual and auditory) and two output (manual and speech) configurations. The tracking task is assumed to be highly dependent upon the response-related resources, whereas the transformation task is assumed to rely more heavily on the perceptual/central-related resources. Both tasks presumably require spatial processing. Further, the four transformation-tracking pairs differ from each other in their requirement for the different input and output resources. Thus, along a continuum of degree of structural similarity, the dual tracking tasks (which have essentially identical task structures) can be placed on one extreme and the transformation-tracking pair that employs an auditory input and a speech output for the transformation task (having no common input or output modalities between the two tasks) on the opposite extreme. Time-sharing efficiency will be assessed by the degree of dual task interference. In accordance with the previous findings, timesharing efficiency is expected to be highest when the primary tracking task is paired with the auditory/speech transformation task.

Method Subjects Twenty-four paid right-handed male participants, ages 18 to 36, were randomly divided into two groups: the predictable difficulty group and the unpredictable difficulty group. Half of the subjects in each group were college students with an average age of 21-3 and the other half were pilots with an average age of 28"8. Of the pilots, two had only a private licence rating, the rest were all instrument rated and all but one had a commercial pilot licence, an instructor pilot licence, or both. Total flight time for the pilots ranged from 120 hr to 2000 hr with a mean of 863 hr. Five of the pilots were not flying professionally at the time the experiment was run.

Tasks

Tracking task (TR). The tracking task was a one-dimensional, first order compensatory tracking task. The tracking display was driven by a band-limited Gaussian disturbance input and was presented on a 10'2 cm by 12'7 cm Hewlett-Packard Model 1330a CRT. Subjects were asked to keep a laterally moving cursor as close as possible to a vertical stationary centre line via an Alphacontrol Model 101-EPST springcentred joystick. The system control dynamics and the system output were governed by a PDP 11/34 computer. The tracking difficulty parameter was the cut-off frequency of the disturbance input. The primary task difficulty was either constant with a cut-off frequency of 0.5 Hz throughout the trial, or varied dynamically within a trial. The time-varying difficulty function was made of a series of ramp increase and ramp decrease of the cut-off frequency between 0.3 Hz and 0'7 Hz. Each ramp was of either 10 s or 20 s duration such that there were two constant rates of difficulty changes within a trial. There were four increasing ramps, four decreasing ramps, four constant 0'7 Hz, and four constant 0-3 Hz segments in each variable difficulty function. Four such variable difficulty functions were generated. The secondary tracking task had a constant difficulty with a cut-off frequency of 0-5 Hz. Tracking error was sampled every 50 ms. A two-second sliding window was used to obtain 199 smoothed Root Mean Square Error (RMSE) per trial. An averaged RMSE was calculated for each trial for the data analyses below.

Transformation task (TM). The stimuli for the transformation task were the eight directions on a compass rose, presented one at a time either visually (V) or auditorily (A). Visual stimulus appeared as a 5 mm long tick mark for 1 s in one of eight possible locations representing each of the eight directions. Auditory directions were represented by three tones presented to either or both ears. A high tone (5000 Hz) presented to both ears represented the direction north, a high tone to the right ear only represented northeast, a medium tone (1600 Hz) to the right represented east, a low tone (600 Hz) to the right represented southeast, and so on. The tones were generated by a tone generator controlled by a Z80A microprocessor interfaced to the PDP 11/34. Each tone was presented for 400 ms at about 69 dB SPL. Subjects were to respond with the next clockwise direction. For example, the appropriate response to the stimulus 'north' would be 'northeast'. Responses were made either manually (M) via eight push-buttons arranged in a circle or by speech (S) via a Votan VTR 6000 voice recognition unit. An upper limit in response time was imposed to maintain the flow of the stimuli throughout the trial and to discourage subjects from ignoring the transformation task during the dual task trials. Performance measures included averaged reaction time (RT) and percent error for each trial. The speech errors had been adjusted for machine recognition errors before the percentage was calculated. Dual task. The primary task was always a tracking task and the secondary task was either another tracking task or a transformation task. Dual tracking (TR-TR) displays were placed one above the other on the CRT screen. The tracking display subtended a visual angle of approximately 4 degrees

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Ergonomics in aviation horizontally. In the transformation-tracking conditions (TM-TR), the visual stimuli were presented above the tracking task, around the centre of the screen. The vertical distance between the transformation and the tracking display subtended a 3-degree visual angle.

Design A mixed design was employed with two groups of subjects performing the same tasks for ten 60- to 90-minute sessions. The only difference between the two groups was the predictability of the difficulty changes. The predictable group had the same variable difficulty function throughout the experiment whereas the unpredictable group had any of four different functions chosen at random for each trial. Furthermore, the second half of the variable function used by the predictable group was a repetition of the first half of the function. The students and the pilots were divided equally between the two groups. The experiment began with four sessions of single task training, followed by three sessions of dual task training. These training sessions were necessary to allow performance to stabilise before the major experimental manipulations were introduced. Single task performance was assessed periodically throughout the experiment. In Session 8, the secondary task paradigm was adopted and the priority instructions to maintain a constant level of primary task performance throughout the trial regardless of the difficulty fluctuations were introduced. On-line performance feedback and a performance scoring system were also introduced in Session 8. There were three sessions performed with the priority instructions. Procedure Each session typically began with some tone training and voice recognition training. Each experimental trial was 200 s in duration. Subjects were given a short break about every six trials. The same general task sequence in each session was followed by all subjects. The order of appearance of the four modality configurations of the transformation task (VM, AM, VS, AS) was randomised for each session. The time-varying function used was randomly chosen for each time-varying difficulty trial for the unpredictable group. Subjects in the predictable group were shown the pattern of difficulty changes at the beginning of the experiment. In the dual task sessions, the TR-TR tasks and the TM-TR tasks were performed on separate blocks. The order of appearance of these two blocks was counterbalanced across sessions. Prior to Session 8, subjects were asked to perform the two tasks as well as possible. Verbal feedback on the average performance was given to the subjects at the end of each trial. When the secondary task paradigm was introduced in Session 8, subjects were asked to allocate their attention to the two tasks proportionally to the primary task difficulty, so as to maintain a constant level of primary task performance throughout the trial. Subjects were provided with an individually determined performance standard at which their primary task performance should be maintained. The subject's best single tracking task performance in the time-varying and constant (0-5 Hz) difficulty conditions served as the primary task performance standard for the

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corresponding dual task conditions. That is, subjects were asked to maintain their primary task performance as close to their single task level as possible. This demanding standard level was necessary to ensure that the subjects were devoting all their available resources to the two tasks at hand. Instructions and performance scoring also encouraged the subjects to maximise their secondary task performance but it was emphasised this was a less important objective. The secondary task performance standard was thus less stringent and was determined by the subject's own best left-hand tracking and best transformation task performance in the corresponding dual task condition. The secondary task performance standard was provided to discourage the subjects from abandoning the secondary task entirely during the dual task trials.

Results Single task performance Single tracking task. Single tracking tasks from Sessions l, 2 and 4 were analysed in a Session x Background (pilots vs students) x Predictability x Bandwidth ANOVA. Session 3 was not included because it was predominantly a transformation task training session. Results show that increasing the bandwidth reliably increased the tracking RMSE at the 0.001 level (F(2,40) = 7.01). The background main effect was not significant. Thus the difficulty manipulation of the tracking task seemed to be effective, and, on the whole, the students did not differ from the pilots in their tracking ability. Of all the single tracking conditions, the time-varying and the constant 0-5 Hz tracking conditions were of particular interest because they served as the baseline from which dual task interference was assessed. These two conditions were analysed in a Session x Background x Predictability x Dynamics (variable vs constant difficulty) ANOVA. The variable RMSE was found to be significantly higher than the constant RMSE (F(1,20) = 39.01, p < 0.001 ). A significant Background x Dynamics interaction was obtained (F(1,20) = 10-22, p < 0,01), showing that the students had lower RMSE than the pilots under the constant difficulty condition only. A Session x Predictability x Dynamics interaction (F(5,t00) = 5.25, p < 0.001) showed that the dynamics effect was practically eliminated after the first four sessions for the predictable group, but not for the unpredictable group until the last experimental session. The background effect was not significant. Single transformation task. A Session x Background x Predictability x I/O Modality ANOVA shows that the RT was significantly different (/7(3,60) --- 119-34, p < 0-001) across the four transformation tasks. The VM condition was the fastest (mean RT = 822 ms), followed by the VS (1103 ms) and the AM (1326 ms) conditions, and the AS condition was the slowest (1799 ms): Since it is inevitable that a speech response would take longer to complete than a finger press, the RT trends reported cannot be interpreted strictly as the amount of time needed for cognitive processing. Thus, for comparison purposes, RT decrement (the difference between a dual task score and a corresponding single task score) instead of RT was used in the dual task analyses below. Students and pilots did not differ significantly in their RT

Ergonomics in aviation to the transformation stimuli. The input and output effects were separated in the next ANOVA: Session x Background x Predictability x Input x Output. The main background effect was again not significant, but there was a reliable Background x Output effect (F(1,20) = 25-94, p < 0-05) with the pilots being slower in the manual conditions but faster in the speech conditions. This interaction can perhaps be attributed to the pilots' familiarity with radio communication. The Background x Input effect was not significant. Performance accuracy was analysed in a Session x Background x Predictability x I/O Modality ANOVA. Results resembled that of the corresponding RT analysis. There was no background effect and the error rates for the different conditions were significantly different from each other (F(3,60) = 32"56, p < 0-001). The VM condition had the lowest error rate (mean = 2-2%), followed by the VS (2-9%) and the AM (7-6%) conditions, and the AS condition had the highest error rate (9"0%). That the same trend was observed in the speed and accuracy measures strongly suggests that the four transformation tasks were not equally difficult at the single task level. The difficulty in translating the different tones into the appropriate directions was at least partially responsible for the slower RT and higher error rate for the auditory conditions. Thus the other reason for using the RT decrements instead of the RT data in the dual transformationtracking analyses was to eliminate the inherent differences between task conditions at the single task level. In summary, except for ti~e Background x Dynamics effect in the tracking data (showing that the students had better performance than the pilots in the constant but not in the variable condition) and the Background x Output interactions in the transformation data (showing that the pilots had a speech advantage), no major differences between the students and the pilots were observed under the single task conditions. Differences obtained under the dual task conditions would therefore not likely be due to a sampling difference of the two groups of subjects.

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particularly for those pairs that shared the same mode of response (TR-TR, VM-TR, and AM-TR). With the exception of a significant Session x Background interaction (F(3,60) = 2"97, p < 0.05), no other significant effect involving the background variable was obtained. The

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Ergonomics in aviation Session x Background interaction was due to a continued improvement in the pilots' primary task performance over the priority instructions sessions (Sessions 8-10), despite starting at a higher level of error in Session 5. In contrast, although the students' primary task error dropped markedly between Sessions 7 and 8, it did not decrease any further beyond Session 8. The primary task performances of the pilots and the students were, however, not reliably different when only data from Sessions 8 and 10 were analysed (see Fig. 2).

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Secondary task performance. The secondary task performance of the tracking and the transformation task were examined separately because different dependent measures were used. A five-way ANOVA was performed on the dual tracking data: Session x Background x Predictability x Dynamics x Priority (primary vs secondary task). Two significant main effects were obtained: session (F(5,100) = 17-12, p < 0.001) and priority (F(1,20) = 151.98,p < 0.001). The Session x Priority interaction was reliable at 0.001 level (F(5,100) = 39-11), showing a marked decrease in the primary task error and a corresponding increase in the secondary task error when the priority instructions were introduced. A reliable Session x Background x Priority interaction (F(5,100) = 2.23, p = 0-06) showed that, in response to the priority instructions, the pilots' secondary task error did not increase quite as much as the students', even though the primary task errors for both groups of subjects decreased to about the same level (see Fig. 3). This could suggest that the pilots did not have to withdraw as many resources from the secondary task as did the students. No other significant effects involving the background variable were obtained.

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observed across the four transformation tasks. By the end of the experiment, the initial {session 5) RT decrement was cancelled for all but the VM condition (the only condition that had the same input and output morality as the timeshared tracking task). The Session x Task interaction was also significant (/'(3,60) = 24.27, p < 0.001). It is worth noting that the pattern of these dual task results (Figs. 1 and 4) does not seem to be predictable directly from the single task performance, where the visual conditions dearly had superior performance. The percentage of error for all four tasks also increased between Sessions 7 and 8 but returned to the prepriority instructions level towards the end o f the experiment. The Session x Task interaction was si~ificant at 0-001 level (F(3,60) = 24-27). The dynamics and predictability main effects were not significant for either dependent measures and neither was the background main effect (see Fig. 5). Effects of the input and output morality were examined separately in the next two ANOVAs (se=don x Baelqgound x Predictability x Dynamics x Input x Output). For the RT decrement, there was a significant output main effect (/7(1,20) = 6.77, p < 04)2) with the manual responses

Ergonomics in aviation

consistently having a larger RT decrement than the speech responses. No significant input effect was obtained. Results obtained with the percent error were similar in the sense that manual responses (mean = 6"3%) had a slightly higher, though not statistically significant, percent of error than the speech responses (5-2%; F(1,20) = 3'75, p = 0'10). However, the visual conditions (2"8%) had significantly tess error than the auditory conditions (8.7%; F(1,20) = 36"61, p < 0-001). A significant Session x Output interaction shows a marked increase in errors for the manual conditions (4"5%) in response to the priority instructions as opposed to 1% increase for the speech conditions. An interesting three-way interaction (Background x Dynamics x Input) shows that pilots were better than the students with the auditory stimuli under the variable condition (F(1,20) = 14-97, p < 0"001). There was no evidence for a speed-accuracy trade-off. In summary, no background main effect was detected in any of the ANOVAs performed on the dual task data. The Session x Background interaction in the primary task and the Session x Background x Priority in the secondary tracking performance suggest that the pilots were more efficient in their time-sharing performance. The pilots also seemed to have a slight advantage with the auditory stimuli under the variable difficulty condition, but no other significant dynamics or predictability effects that are related to the background of the subjects were detected.

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Structural effects on time-sharing performance In accordance with previous research, a higher degree of dual task interference was observed with the task pairs that place heavy demand on the same resources as delineated in the structure-specific resource model. First, the primary task performance was closer to the standard level for those pairs that did not compete for the same output resources. Second, the secondary tracking error remained high after the priority instructions were introduced regardless of practice. In contrast, the structurally less similar transformation performance decrement, though initially increased as a result of the priority instructions, later decreased with practice. Further, among the four transformation-tracking pairs, the VM condition (which had the same I/O modalities as the primary task) suffered the greatest RT decrement and improved the least with practice, despite its fastest time and highest accuracy at the single task level.

Comparingthe

students and the pilots In general, the difference observed in the time-sharing performance between the students and the pilots was not overwhelming. In none of the ANOVAs performed, be it on tracking RMSE, RT or accuracy performance, was there a significant background main effect. The only evidence which suggested that the pilots were more efficient in their timesharing performance was the Session x Background x Priority interaction in the dual tracking condition (Fig. 3). This interaction suggests that the pilots did not have to withdraw as many resources as the students from the secondary task in order to protect their primary task performance. There was, however, no indication that the pilots' time-sharing performance was any more immune to structural interference than the students' (Figs. 2 and 5). The predictions from the structurespecific resource model thus seemed to apply equally well to the pilots and the students. Certainly, more studies with different sub-populations should be conducted, but the present results provide no evidence that time-sharing skills decrease the extent of structural interference as hinted in Damos and Wickens (1980).

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Comparisons of the pilots' and college students' secondary task performance across the four transformation-tracking conditions

While the present data suggested that the pilots' timesharing performance was more efficient than the students' in the dual tracking condition, the evidence that the pilots in general have better time-sharing skills was not compelling. The lack of a significant difference between the two groups' responses to the structural interference among the timeshared tasks has three important implications. First, it lends further support to the structure-specific resource model proposed by Wickens (1980). Second, that the structural effects on time-sharing performance were not easily altered by extensive practice supported the validity of laboratory results that were obtained mostly with college students, and its application in operational settings. Third, while the level of time-sharing performance can be improved with training and practice, the structural effects perhaps can only be minimised by proper system and multitask designs.

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Ergonomics in aviation Acknowledgements The research was conducted while the author was a National Research Council Research Associate at NASA Ames Research Center, Moffett Field, California. The author would like to thank E.J. Hartzell for providing the experimental facilities.

McLeod, P.D. 1977 Quarterly Journal of Experimental Psychology, 30, 83-90. A dual task response modality effect: Support for multiprocessor models of attention.

Norman, D.A., and Bobrow, D.G. 1975 Cognitive Psychology, 7, 44-46. On data-limited and resource-limited process.

References Ackerman, P.L., Schneider, W., and Wickens, C.D. 1984 Human Factors, 26, 71-82. Deciding the existence of a time-sharing ability: A combined methodological and theoretical approach.

Crosby, J.V., and Parkinson, S.R. 1979 Ergonomics, 22, 1301-1313. A dual task investigation of pilots' skill level.

Damos, D.L., and Wickens, C.D. 1980 Acta Psychologica, 46, 15-39. The identification and transfer of time-sharing skills.

Gopher, D., Briclmer, M., and Navon, D. 1982 Journal of Experimental Psychology: Human Perception and Performance, 8, 146-157. Different difficulty manipulations interact differently with task emphasis: Evidence for multiple resources.

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North, R.A., and Gopher, D. 1976 Human Factors, 18, 1-14. Measures of attention as predictors of flight performance. Ogden, G., Levine, J., and Eisner, E. 1979 Human Factors, 21,529-548. Measurement of workload by secondary task. Tsang, P.S., and Wickens, C.D. 1984 Proceedings of the 20th Annual Conference on Manual Control, Vol II (Washington, DC: National Aeronautics and Space Administration), pp 305-318. The effects of task structures on time-sharing efficiency and resource allocation optimality. Wickens, C.D. 1980 The structure of attentional resources. In R.S. Nickerson (ed), 'Attention and Performance VIII', pp 239-257. Lawrence Ertbaum, New Jersey.

Wickens, C.D., Mountford, S.J., and Schreiner, W. 1981 Human Factors, 23, 211-229. Multiple resources, task. hemispheric integrity, and individual differences in time-sharing.