The time-scaled arrow diagram (TAD) is used to analyze, plan, and schedule projects in order to meet a target date at a minimum cost. In the process, the TAD helps to deter- mine which project activities are “critical” in their impact on project completion time to provide focus for the project team. As its nominal definition states, TAD is an activity on arrow tool, and is the only critical path method that is displayed against a time scale (Figure 6.5).
Like other schedule development tools, the TAD helps you identify the total comple- tion time, understand the sequencing of activities, identify when resources are needed, monitor activities that are critical, and assist the project manager in measuring progress.
Developing a TAD
Building a TAD is an exercise that requires endurance and discipline, unfolding through several major steps. As with all schedule development tools, the first thing to do is to determine the level of detail required and identify the project activities. Typically, this is part of scope planning. We include it here in order to provide an integrated view of this tool’s development.
The TAD’s quality is heavily dependent on solid information about project scope, team member responsibilities, availability of resources, and the higher-level schedule management system.
THE TIME-SCALED ARROW DIAGRAM 163
1.01 Select Concept 1.02 Design Beta PC 1.03 Produce Beta PC 1.04 Develop Test Plan 1.05 Test Beta PC 2.01 Design Production PC 2.02 Outsource Mold Design 2.03 Design Tooling 2.04 Purchase Tool Machines 2.05 Manufacture Molds 2.06 Test Molds 2.07 Certify PC 3.01 Ramp Production
Work Packages/Tasks Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan
1.01
1.02 1.03 1.04
1.05
2.01 2.02 2.03
2.04 2.05 2.06
2.07 3.01
1.01
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1.04 1.05
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2.03 2.04
2.05 2.06
2.07 3.01
Figure 6.5: Example of a Time-Scaled Arrow Diagram
Clearly, you need to understand the project scope in order to schedule the project activities. The purpose of knowing who is responsible for certain activities is to indi- cate who will schedule and then manage the activities. For this to be possible, project schedulers need information on resource availability. Guidelines for how to develop and maintain a TAD will come from an organization’s scheduling management system.
Determine the Level of Detail and Identify Activities
How large or small a project is influences the number of activities in a TAD. An example may shed more light on this issue. Large new product introduction projects typically include 300 to 500 activities, with durations between three and five weeks. The intention is to provide just enough information—neither more, nor less—than what one needs to direct and monitor project work of a certain size and complexity. Providing more or less information than necessary may either overload the team or deprive it of essential information, respectively.
Once the level of detail is chosen, identify activities that have to be performed to complete the project. As is the case with all types of scheduling tools, an excellent way to do this is to employ the WBS, perhaps the most systematic and integrated way of activity identification. Activities necessary for developing a TAD are those needed to produce work packages, the lowest-level elements of the WBS.
When the activities are identified, concentrate on the level of detail that was selected earlier. If the actual number of activities is smaller than the targeted number, resume breaking down larger activities. Should the actual number exceed the target number, some similar activities can be merged to reach the desired level of detail.
Sequence Activities
Sequencing involves determining the logical flow of project activities and establishing the dependencies between the activities. This means putting activities into a specific
order by determining an activity’s immediate prerequisite activities, called predecessors, and leaving no loose ends (see “Loose Ends May Mislead the Team”). As explained in the section on the CPM diagram, some dependencies will be hard or logical; others will be soft or preferential. Both types can be used to create overlapping activities, of course, for the purpose of fast-tracking a TAD.
The TAD makes available a number of ways to represent dependencies: finish-to- start (FS), start-to-start (SS), finish-to-finish (FF), and start-to-finish (SF).13 To each of those we may specify a lead/lag factor in order to accurately define the dependency.
(See the examples in Figure 6.6.)
How much are these dependencies really used in practice? How much do we need them? Traditionally, FS has been used extensively. The FF and SF dependencies, how- ever, historically have seen little use. Generally, the SS dependency has gained huge popularity in businesses competing on faster project cycle times. In short-cycle-time project environments, the SS dependency is what is needed to fast-track a project.14 Its major benefit is in allowing parallel work. Consider, for example, developing a new computer, where software development has an SS with lag dependency with hardware
FS – Activity B can start only when activity A is finished. When you add a two-day lead/lag, activity B can start only two days (lead/lag) after activity A is finished.
B 8 A
5
B 8 A
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FS FS with lead/lag factor
(2)
SS – Activity B must not start before activity A starts. Adding a lead/lag, activity B must not start before activity A has been in progress for at least two days.
B 8 A 5
B 8 A 5
SS with lead/lag factor SS
(2)
FF – Activity B must be finished at the same time as activity A. With a lead/lag, activity A must be completed at least two days before activity B can be finished.
B 8
A 5
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SF – Activity B cannot be completed before activity A starts. Inserting a lead/lag, activity B cannot be completed before seven days from the start of activity A.
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SF SF with lead/lag factor
A 5
with lead/lag factor
Figure 6.6: Types of Dependencies between Activities
THE TIME-SCALED ARROW DIAGRAM 165
development. To start its work, the software team needs at least a hardware design, and once they get it, they can carry on with their work in parallel with the hardware development.
Loose Ends May Mislead the Team
We frequently see TADs with loose ends, including arrow tails and arrowheads that are not connected to other activities. When we ask project managers why, we often get an answer like, “I only want to show the critical path and depen- dencies on it. Other paths and their dependencies are not important to me.”
This is a risky practice. To determine the critical path, the team has to eval- uate all paths with properly connected activities. If there are loose ends, the team may not see the real critical path. Then the whole purpose of having a TAD—focus on really critical activities—is defeated.
Assign Resources and Estimate Activity Duration
The heart of schedule development is resource allocation and scheduling. Although it was touched on in the CPM section, we repeat it here. The first rule of resource allocation is to identify resources necessary to successfully complete activities. For example, you may need a cost estimator and precisely 80 hours of his work (effort) time. Assume now that the estimator is shared by this and two other projects. Look at the company’s work calendar as well (50 hours per week maximum; no work on Saturdays and Sundays). All this information tells that the estimator will need to spread his 80 hours’ worth of work over 10 calendar weeks (calendar time). Reiterate steps of identify resources, determine work time, and convert it to calendar time for all remaining activities. Estimating activity durations can be tricky, especially in multiproject environments (see “Switchover Time Adds to the Schedule Inaccuracy”).
Switchover Time Adds to the Schedule Inaccuracy
Some 90 percent of projects are implemented in a multiproject management environment. This means that a practice of having project managers run multiple projects at a time, anywhere from two to ten, is widely accepted.15 While such an approach provides outstanding benefits in terms of better management, it also generates a unique problem that calls for very meticulous scheduling of the projects. In particular, the problem here is the switchover time. When the project manager switches from one project to another, she needs switchover time to align her thinking and get into the new project, physically and mentally.16
(continued)
Since team members also operate as members of multiple project teams, they suffer from the same problem. As the projects grow in complexity, so does the switchover time.17Clearly, this time is a loss in a busy day of a project manager and team member. For example, some experts indicate that this loss may be up to 20 percent of the project manager’s or team member’s time, when involved in four projects at a time. The real problem then is that this switchover time loss typically is not taken into account in scheduling multiple projects. As a consequence, project schedules are notoriously optimistic and inaccurate.
There are at least two strategies to deal with the problem. One is to reduce the available monthly work hours of a multiproject person used for scheduling purposes by the corresponding switchover time loss. Another is to increase the estimate of a multiproject person’s work hours for a specific project by the corresponding switchover time loss. These strategies are not attractive, but they are necessary for the realistic development of any project schedule or dependency diagram.
Draft the TAD
Draw activities as arrows, connecting them to one another—arrowhead to arrow tail to indicate the sequence of dependencies (see Figure 6.6). In that manner, all immediate predecessors of an activity lead to the beginning point of the arrow tail, while all imme- diate successors stem from the arrowhead. Thus, the beginning point of the arrow tail becomes an event, where all activities leading to the point are completed. Obviously, a TAD can be drawn in two different formats (see Figure 6.6 and the example that follows,
“Cascade- versus Spine-Formatted TAD”).
Cascade- versus Spine-Formatted TAD
Cascade
One zone, one activity. A zone is a horizontal swath or strip across a TAD print- out. The cascade format allows only one activity per zone.
Why called cascade? A well-arranged succession of activities, one per zone, appears like a cascade of events.
Less complex. The cascade resembles a Gantt chart, a simple-looking tool, which creates a sense of lower complexity that is easier to apply.
Less practical. Because of one activity per zone, a larger TAD may require many sheets to print it and large wall space to post it.
Spine
One zone, multiple activities. The TAD printout allows multiple activities per zone.
Why called spine?Activities are symmetrically arranged around a central path, usually the critical path, resembling a spine of the network.
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More complex. The appearance of the spine is much like any other network, something that looks scarily complex to some project managers.
More practical. Because of multiple activities per zone, you can print a larger TAD on a single sheet of paper and post on it on a small wall space.
Identify the Critical Path
Normally, a TAD shows a number of different paths—that is, sequences of dependent activities. The paths can be used in two ways to find the critical path. First, you can visually find a path composed of activities without float—no complex calculations nec- essary. Among all network users, this convenience is available only to those who practice TAD. Adding up the times for all activities in a path (as we did with CPM) will tell how long it is. As a reminder, the critical path is the longest path in a TAD, which indicates the min- imum time necessary to complete the entire project. Second, you can find the critical path with the forward/backward pass procedure and calculate total and free float, as explained in the CPM section featured earlier in this chapter.
Using a TAD
Like any network diagram, the TAD’s original targets were large, complex, and cross- functional projects. The TAD is well suited to such projects because of the ease with which it handles a large number of activities and their intricate dependencies, directing our attention to the most critical activities. While it is still applied for this purpose, a large number of project managers have used the TAD for medium-size and small projects (see
“Tips for TAD”). In this case, a TAD is typically drawn in a cascade and often called a “Gantt chart with links.” Perhaps more than anything else, this format facilitated the growing popularity of the TAD.
A sizeable TAD can be used along with a Gantt chart to provide focus on the day-to-day project work. In particular, we can take out from TAD those activities due in the next week or two, show them in the Gantt chart format, and have their “owners”
use them as short-term outlook schedules. This provides a balance in focusing on both the big picture of the project with TAD and daily work details with Gantt charts.
Tips for TAD
■ If you need to fast-track your schedule, use SS with or without lags. Be prepared to see and manage 40 to 50 percent critical activities in the schedule.
■ Building on the similarity between a cascade-formatted TAD to a Gantt chart, spread the use of the TAD in all smaller projects. This will significantly enhance the quality of scheduling.
■ Add major milestones to a TAD to help vital events serve as beacons in the sea of activities.
■ Insist on the use of template TADs to boost quality and productivity of scheduling.
Benefits
The TAD offers a unique benefit not available to any other network diagram: the ability to read directly off the schedule’s time scale when the project and each activity starts and ends, as well as the total float. Like other network diagrams, the TAD helps identify the total completion time, understand the sequencing of activities, ensure resources when necessary, monitor those that are critical, and measure progress (or lack of it).
The TAD’s graphical appeal and intuitive logic provide additional utility to a project manager. The TAD’s unambiguous sequence of work, supported by the timescale, is eas- ier to clarify than any other network diagram. Data calculations are not difficult and can be handled readily and quickly by personal computers. Additionally, the TAD exhibits the dependencies between constituent activities of a project simply and directly. This helps fathom the order of activity execution.
Perhaps the highest value comes from the TAD’s focus on priorities; TAD directs our mind on the vital few activities of critical importance to the project completion date. The outcome is higher accuracy and, later, precision of schedule control.