Task analysis
5.5 Data analysis
5.5.1 Introduction
There are a wide range of methods that can be used to assist an analyst to examine the data that have been collected and either to identify any potential problems or to assess whether particular criteria can be met.Each of these analysis methods is appropriate
for different situations and it is beyond the scope of this chapter to provide detailed descriptions of each method.Readers who do wish to read more detailed descriptions of the methods are referred to Kirwan and Ainsworth [4] which provides a more comprehensive discussion of several task analysis techniques together with some case studies that show how they were used.
In order to provide the analyst with an indication of the way that potential issues can be examined and the choices that are possible, seven general application areas have been chosen and for each of these, some potential analysis methods are described.
It must be stressed that whilst the methods have been described in the context of specific applications, several of the methods could also be applied very effectively to help resolve other issues. It must also be emphasised that often the underlying issues are so evident from the basic data, that there is no need to undertake further analysis of the decomposition data in order to clearly identify potential human factors problems.
5.5.2 Analysis of the working environment
Task performance on both physical and mental tasks can be influenced by the environ- mental conditions under which the tasks have to be undertaken, and in more hostile environments there is also a health and safety implication. Therefore, it is often necessary to obtain some information about lighting, noise levels, or thermal condi- tions in which tasks may have to be undertaken. For some specific tasks it may also be necessary to consider other potential environmental hazards, such as vibration, motion or radiation.
Providing that there is a system already in existence, the following environmental assessments can be made:
• Environmental surveys to establish the thermal conditions, such as air movement, humidity and temperature by taking the appropriate measurements, such as wet and dry bulb temperatures, radiant temperatures, etc.
• Lighting surveys to measure the lighting levels for particular tasks and to identify potential sources of glare. For many applications it will also be necessary to assess the adequacy of emergency illumination. It should be noted that these measurements can be taken in a pre-operational environment.
• Noise surveys to measure the background sound level in dB(A) under typical working conditions to ensure that hearing protection will be provided if there is a risk to health. Such surveys should also measure the sound levels of communi- cations and of auditory alarm signals to ensure that these are sufficiently above the background.
Alternatively, the analyst will have to rely upon statements about the pro- posed environmental conditions, or else to define the conditions that should be provided. If necessary, this information can then be assessed against guidance pro- vided in ergonomics checklists. The checklists provided in Section 5.8 of MIL STD1472 [5] and in Sections 12.1.2 and 12.2.5 of NUREG-0700 [6] provide very comprehensive guidance information about the lighting conditions, noise levels or
thermal conditions.More comprehensive design guidance is provided in Chapter 9 of this book.
5.5.3 Analysis of the workstation design
To check that workers can be accommodated at their workstations and that they will be able to reach all the necessary controls, it will first be necessary to identify the relevant dimensions. There may be two sets of dimensions that will be identified.
The first of these will be the general workstation dimensions that must be considered in order to ensure that the largest(95th percentile)users can fit into the workplace and the smallest(5th percentile)will be able to reach common controls, such as a mouse or keyboard. In many cases the task descriptions will also identify that other controls or displays may be required, and it will then be necessary to ascertain whether these can be used from the normal operating positions. The analyst must then obtain the relevant workplace measurements, either from drawings that are provided, or by direct measurement. The workplace dimensions can then be compared with standard anthropometric data, such asPheasant [7], after adding allowances for clothing, such as increasing standing height by 25 mm for males wearing shoes. At the same time, the operating forces on the controls can be checked against ergonomics guidance that is provided inPheasant and in other standard ergonomics texts.
If any of the tasks require manual handling, the NIOSH Lifting Equation (see Waterset al. [8])can be used to assess whether these tasks can be undertaken safely.More comprehensive design guidance is given in Chapter 9 of this book.
5.5.4 Analysis of information requirements
An understanding of the information requirements for each task may be useful either to define the interface requirements, or to identify the knowledge and skill requirements.
Perhaps the most effective way to examine these issues is to produce some form of diagrammatic representation of selected tasks that indicates the relationships between particular parameters or activities. This can be done by drawing process charts, but these tend to be too coarse-grained for most task analysis purposes.Other possibilities are to produce input–output diagrams [9] orPetri nets [10]. The former use block diagrams to show the information transitions that occur during tasks, whilst the latter examine the possible state changes that could occur in key parameters and trace their impact. However, this section will be limited to short descriptions of information flow diagrams and functional block diagrams.
Information flow diagrams, which are also known as decision–action diagrams, or decision–action–information diagrams, depict the information flows and trans- formations in a system.On these diagrams, actions are presented in rectangles and decisions are shown within diamonds. After each decision box the possible outcomes are labelled on the exit lines, usually as binary choices, though they can also be multiple choices. An example of a decision–action diagram is presented in Figure 5.4 to illustrate this. This particular example is based upon a continuous process control task and so it has a closed loop structure. However, for other applications the diagram may well be much more linear.
Close valve Open valve Allow to stabilise Monitor flow
Is flow alright?
Adjust rate Check tank
level Open bypass Re-align
Is cooling pump rate OK?
Initiate slow cooldown
Isolate and re-align circuit
Start emerg pump Is level
falling?
Is temp rising?
Is pump
running? Start pump
Too High
Too No
No No
No Low
Yes
Yes See A2
Yes Yes
Yes
Figure 5.4 Decision–action diagram
These diagrams provide an indication of the information requirements and they are particularly useful because they focus attention upon decision-making activities.
Wherever an analyst identifies any decision-making activity, it is recommended that this should be examined carefully to ascertain whether all the parameters that could influence a particular decision have been identified. If there are other important influences, these can either be added to the diagram, or else the component decisions can be shown elsewhere. Where it is appropriate to provide a separate diagram for a particularly complex decision, it is recommended that the relevant decision box on the parent diagram should be surrounded by pecked lines and a reference to the more detailed diagram should be given, as is illustrated on the ‘Is cooling rateOK?’ box in Figure 5.4.
Functional block diagrams provide a representation of the main functional require- ments, in which the different functional requirements are presented in separate boxes, with arrows linking the boxes to show the sequential relationships between the func- tions and subfunctions that have to be undertaken to complete a task. ANDandOR gates are used on these diagrams to describe more complex sequential relationships.
An example of a simple functional block diagram is shown in Figure 5.5.
Each function represented on the diagram should be uniquely numbered, with lower level subfunctions being hierarchically related to their parent function. It will seldom be fruitful to proceed past four levels in this hierarchy. It is important to stress that this numbering is based upon functional relationships and as such it may differ from the normal HTA numbering. For this reason, the numbers are separated from the rest of the box by a horizontal line.
For some purposes it will not be possible to present all the functional blocks on a single page and so a set of diagrams will be necessary. Such sets of diagrams can be organised either sequentially or hierarchically. In the former case, the functions that are undertaken will be shown on the first diagram and subsequent diagrams will show functions that are undertaken later. For a hierarchical arrangement, the main functions will be shown on the first diagram and each of the main functions will then be shown in greater detail elsewhere.
1
2
4.1
7
6 Use first
line cooling 4.2
5 Locate leak Use second line cooling
Monitor cooldown
Isolate leak 3
Detect and diagnose fault
Trip reactor
Ensure adequate electric supplies
OR OR
AND
AND
Figure 5.5 Functional block diagram
5.5.5 Analysis of task sequence
Link analysis and operational sequence diagrams are specifically used to examine task sequences in terms of the use of interfaces or communications links. Both techniques can be used either to present information schematically, or they can be based upon more accurate plans or drawings that maintain the topographic relationships between items.
Link analysis was originally used to investigate the social relationships between people in group situations. This was done by representing each person as a circle on a schematic diagram and then drawing a separate line between the appropriate circles each time that two people interacted with each other in some way. Thus, the number of lines provided an indication of the frequency of interpersonal interaction or communi- cation between individuals. Clearly, this technique can be applied directly to working situations to look at communications between team members during various tasks.
It can also be adapted to assess the relationships between different instruments or parts of an interface, by using a schematic or topographical diagram of the interfaces and then drawing a link between items on this drawing when the focus of attention changes. This is known as a frequency link analysis diagram, but it is of limited use because it is not possible to determine the task sequence from such diagrams. There- fore, such diagrams are open to misinterpretation. For example, the task context may mean that it is more important to keep together two components with relatively weak frequency links that are used during an emergency, rather than other components with much stronger links that are always used in less critical situations with no time stress.
In order to examine the sequential relationships a continuous line can be drawn to indicate the changes of attention that occur in a task. This is known as a spatial link diagram and it enables an analyst to directly trace the changes between different inter- face elements. This type of analysis is particularly useful for identifying where there is a wide physical separation between interface items that are accessed sequentially
that could cause time stress. It can also identify mismatches between the physical layout and task order, which could lead to steps being omitted or being undertaken in the wrong order. For computer-based systems, spatial link diagrams can be drawn with separate locations on the diagram for different pages, and this provides a useful way to ensure that information and controls that are used together will be presented on the same display page.
Operational sequence diagrams provide other graphical ways to look at task sequences, using some basic symbology to differentiate between different types of operation.Different authors have used different symbology in operational sequence diagrams, but the most widely used symbols are defined in Kirwan and Ainsworth [4], or in original papers on the technique, such as Kurke [11].
There are four types of operational sequence diagram:
• Basic operational sequence diagrams. These categorise each task into one of five types of activity corresponding to the activities listed in the basic symbol set.
Then these are represented as a vertical flow chart using these symbols. This type of representation is sometimes useful for tasks that are undertaken at a number of different locations, but it generally adds little to many applications, such as control room-based tasks.
• Temporal operational sequence diagrams.Effectively these are basic operational sequence diagrams on which the vertical axis is also a timescale. It is considered that timelines are a better way to show this information and so these are not recommended.
• Partitioned operational sequence diagrams. These consist of separate operational sequence diagrams for different aspects of a task that are presented together. For example, a two-person task could be presented as a vertical operational sequence diagram for each person, with a centre column being used to indicate interactions between them. Tainsh [12] has developed a method that he called job process charts, which essentially uses this approach to look at the relation between com- puter tasks and human tasks. In job process charts, the outside columns are used for computer tasks and human tasks, whilst the central column shows the interaction between the two.
• Spatial operational sequence diagrams. These are simply spatial link diagrams to which operational sequence symbols have been added.
In order to produce any form of link diagram or operational sequence diagram it is first necessary to collate the information in some way. Normally this would mean that task decomposition tables could be used directly as the data source. However, it can be helpful to organise this information in some way prior to producing the diagrams. Information sequence tables provide a tabular way to do this, in which the different columns of a table are used to group the information that comes from different sources. Such diagrams can provide a particularly effective way to look at computer-based workstations that comprise several screens, so that screen usage can be optimised. An example of how this approach could be used to examine the usability of a three-screen workstation is shown in Table 5.1.
Table 5.1 Information sequence table
Task Key Screen 1 Screen 2 Screen 3
presses
1.1.1 Check inventory 1 R100 (unassigned) Alarms
1.1.2Ensure conditions 3 R101 T100 Alarms
stabilising T1011
1.1.3 IsolateRHR 1 R102 Alarms
1.1.4 Start four 1 R102 T1011 Alarms
emergency pumps T1012
5.5.6 Analysis of temporal and workload issues
Two persistent issues in task analyses are whether personnel will have sufficient time to successfully complete all of the necessary actions and whether they will have the capacity required to perform their tasks reliably. These two issues are assessed by timelines and workload analyses respectively.
The starting point for the development of any timeline is the collection of data about the time required for each task. This can be done during the task decomposition stage by directly observing task performance, or by obtaining subjective assessments.
In the former case, it will be necessary to ensure that the timings are based upon realistic situations. In particular, there is a risk that if operators are timed whilst they undertake actions that they are prepared for, the resultant times may well be much shorter than would be required when there is some uncertainty, or when there are interruptions from other tasks that are competing for attention. Thus the analyst must endeavour to ensure that the tasks are undertaken in a realistic manner and that where necessary allowances are made for any interruptions or problems that may occur. If subjective estimates of time are being used, it will be necessary to select people with sufficient understanding of the tasks to be able to provide realistic assessments and, if it is feasible, an attempt should be made to validate some of the time estimates against direct measurements.Yet another way to estimate task times is to use standard data, either from psychological models or by timing representative tasks. For instance, rather than timing operators opening every valve on a plant, an analyst may use the time taken to open one specific valve and use the same value throughout a task analysis.
A final consideration that should be made when determining realistic task times, is how to make allowances for the effect of adverse environmental conditions, such as darkness, cold or wind upon the time required. The impact of such conditions is illustrated byUmbers andReiersen [13] who noted that a combination of adverse conditions could increase task times by as much as 70 per cent. Therefore, it has been proposed that task times should be multiplied by an appropriate factor for particular environmental conditions. In Table 5.2, adapted from [14], some weighting factors are given.
Table 5.2 Adverse conditions factors (adapted from [14])
Environmental factor Type of task
Simple manual Walking
Sub zero temperatures, 2 1.5
ice and snow
High winds 1 1.5
Darkness 1 2
Wearing breathing 1.25 2
apparatus
Time (min) 0.00 1.00 2.00 3.00 4.00 5.00 6.00
Criteria not given in procs.
Monitoring/
control
Decision- making
Monitor and wait for local-to-plant tasks.
Need a trend display.
Simple decision.
Identify source of leak.
A checklist would be useful for local- to-plant operator.
Isolate leak and monitor cooling.
Monitor pump startup and effect on cooling.
Decide to provide additional cooling.
Not easy to identify all the valves. Some labels missing.
Re-align cooling system and start emergency pumps.
Task
Others
involved Comments
Make initial diagnosis. Little guidance available.
Super. monitors one train.
2 other Turbine Hall Operators.
1 Turbine Hall Operator.
Discuss with Super.
Straightforward procedural task.
Ensure system isolated and then start pumps on one train.
Figure 5.6 Vertical timeline ( from unpublished data by Ainsworth)
A timeline is basically a bar chart for either single-person or team-based tasks, in which the length of the bars is directly proportional to the time required for those tasks. Figure 5.6 shows a timeline in which the time axis is presented vertically. This has the advantage that ancillary information can be presented as text alongside the timeline, but adding this text also means that the timescale must be relatively coarse.
It is, however, more common to present timelines horizontally. For a single person the tasks for a horizontal timeline are generally listed along the vertical axis, so they appear as a rising, or falling, series of bars, as depicted in Figure 5.7.
For team-based tasks, the horizontal timeline can be modified by presenting all the tasks undertaken by the same person at the same level(see Figure 5.8 for an example). This then enables an analyst to examine both individual tasks and the interactions between team members. In such situations there are often critical points
20 40 60 80 100 120 140 160 secs
Start emergency pumps
Trip main pumps
Start emergency pumps
Re-align cooling flow Balance flows Isolate train B Isolate train A Trip pumps Diagnose problem 14
10 24
24 19
28 18
Figure 5.7 Horizontal single-person timeline ( from unpublished data by Ainsworth) where one team member must complete a particular task before others can continue with their tasks. Therefore, to highlight such dependencies, such situations are shown by using arrows to link the respective bars on the timeline. Where team members work at different locations, it can also be useful to present tasks undertaken at the same location adjacent to each other in the timeline.
Clearly, timelines are particularly useful for assessing time-critical tasks. There- fore, it is recommended that the occurrence of time-critical events should be shown on timelines by vertical lines. This will assist an analyst to identify any time-critical events that cannot be completed within the required time. In such situations there are four strategies that can be adopted to reduce the time required to achieve the required system states.
• Try to avoid less important tasks during time-critical periods.
• Provide job aids or additional automation.
• For team-based tasks, re-organise or re-allocate the tasks to reduce waiting times by other team members.
• Increase the staffing levels for these tasks.
Whilst timelines represent the time required to undertake tasks, they do not provide any indication of the amount of effort, or the workload, involved. There are several subjective methods that can be used to assess workload, the most widely used of which is probably the NASA Task Load Index(TLX)[15].
NASA TLX is based upon subjective ratings made on six different dimensions of workload, namely mental demand, physical demand, temporal demand, effort, performance and frustration level. In the first stage of any assessment the subjects are presented with descriptions of each of these dimensions and they are asked to make pairwise comparisons of the importance to workload of each possible pair of dimensions. This enables individual weightings to be developed for each dimension.
After undertaking a task, the respondents are then asked to rate the workload that