Usability Questionnaire Results - Institute of Aviation - University of ...

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4.6 NASA-TLX Workload ScoresThe average overall workload rating was 33.4 (SD=19.7) for the tabular display, 21.9(SD=21.9) for the one-line display, and 23.5 (SD=23.5) for the contour display. Figure 4-5shows TLX scores for the six dimensions that comprise the overall workload measure for each ofthe display formats. For every dimension except frustration level, the average workload scoresfor the tabular group were higher than either the one-line or contour group. The greatestdisparity between the tabular group and the one-line and contour group was for the temporaldemand dimension where the score was nearly twice the amount for the tabular group. A 3(Group: Tabular vs. One-line vs. Contour) x 6 (TLX Dimension: Mental vs. Physical vs.Temporal vs. Performance vs. Effort vs. Frustration Level) ANOVA was performed to look fordifferences in the amount of workload for the three different display types. However, nosignificant effects were observed.NASA-TLX Dimension ScoresTLX Score706050403020100TabularOne-LineContourMental DemandPhysical DemandFigure 4-5. Workload ScoresTemporal DemandPerformanceEffortFrustration Level5. DISCUSSIONThe current study was conducted to evaluate new visualization techniques for powersystems data. The integrated one-line diagram display was designed using some of these newvisualization techniques that resulted in a display that was both a mimic display and a closespatial proximity display. Color contouring was also examined as an enhancement technique.The performance effects of the integrated one-line diagram display were evaluated by comparingit to a separate tabular display and static one-line diagram in the context of two process controltasks. The first task, detection, required participants to locate voltage value faults in a 30 buspower system. The second task, diagnosis, required participants to integrate information aboutbuses and capacitors to solve the faults. Predictions about participants’ performance of thesetasks with either the separate tabular display and one-line diagram or the integrated one-line29
diagram display were derived from the human factors literature. The results will first bediscussed in terms of the detection task and then in terms of the diagnosing task.5.1 DetectionThe tabular display was predicted to improve performance for the detection taskcompared to the integrated one-line diagram display. This prediction was supported. Thetabular group showed that they were able to acknowledge violations quicker by about 1-2seconds. The speed advantage of acknowledging using the tabular format is best explained onthe basis of the visual search literature. The tabular display depicted bus and voltage valueinformation in column format and hence capitalizes on certain features of perceptualorganization. Since target searching is driven in part by the expectation of where a target will belocated (Wickens & Hollands, 2000), participants were most likely scanning the column wherethe voltage violations would appear. Due to the grouping of the buses and voltage values into asingle column, high redundancy occurred that allowed participants to accurately guess thelocation of other voltage violations once the first voltage violation was found (Garner, 1974). Incontrast, the integrated one-line diagram display lists the voltage values at buses placedsporadically on the screen which are meant in practice to spatially match the physicalrepresentation of the system. These buses of the integrated display were spaced out at widerdistances compared to the column format of the tabular display. The need to search across largergaps between buses served to increase the search times slightly in this type of display (Neisser,Novick, & Lazar, 1964; Drury & Clement, 1978; Teichner & Mocharnuk, 1979). As a resultvisual search time was reduced for the tabular display relative to the integrated display.The column format of the tabular display also led the participants to develop a dominantstrategy for detecting and acknowledging the voltage value faults. A top to bottomacknowledging strategy was used when voltage violations occurred, where participants begandetecting and acknowledging at the top of the column first and then scanned down through thecolumn. This strategy makes intuitive sense and can be explained within a visual searchparadigm. Visual search in displays normally occurs in a serial sequence where each item isinspected in turn (Drury & Clement, 1978; Treisman & Gelade, 1980). When searching throughmenus and vertical columns of data, the serial search often occurs from top to bottom (Somberg,1987). Since all the buses in the presented tabular display were perceptually organized by beingaligned in a vertical column, the easiest strategy for acknowledging violations would have beenstarting at the top and going down the list and clicking all the voltage values in red.The color contouring enhancements for the integrated one-line diagram display wereexpected to offset some of the cost in time for detecting and acknowledging associated with theintegrated display due to the contouring acting as a type of highlighting feature. This prediction,however, was not supported. In fact, the use of the color contouring led to worse detectionperformance on trials with multiple violations.Seeing multiple color contours when faults occurred might have been distracting to theparticipants and interfered with quickly detecting and acknowledging violations. Indeed,contours also did have some effect in terms of strategies used by the operators when detectingand acknowledging. The voltage violation with the largest contour was acknowledged first moreoften in the contour display than for the equivalent voltage violations in the tabular and one-line30
Page 2 and 3: ABSTRACTTraditional power system di
Page 7 and 8: 1.2 Evaluation of Integrated One-Li
Page 9 and 10: Third, false alarms are much more c
Page 11 and 12: 2.2.2.1 Close Spatial Proximity Dis
Page 13 and 14: Furthermore, configural displays ne
Page 15 and 16: calling for precise judgment of one
Page 17 and 18: found that subjects performing a vi
Page 19 and 20: 1974). This reduces visual search t
Page 21 and 22: organized as well perceptually, and
Page 23 and 24: Figure 3-1. Tabular Display280.97 p
Page 25 and 26: 3.6.2 Phase 2: TrialsParticipants i
Page 27 and 28: with multiple violations (M=3.316,
Page 29: Median # of Capacitor Closures per
Page 33: effect on the bus voltages that was
Page 36 and 37: Carswell, C. M. & Wickens, C. D. (1
Page 38 and 39: Pomerantz, J. R. (1981). Perceptual

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