Work packages from the WBS are used to build the activities found in the project network. This segment shows the project manager that activity A must be completed before activities B and C can begin.
Network Computation Process
In our Automated Warehouse example, the early start time for the first activity (activity A) is zero. Next, we see that activity A is the precursor to activities B (Assign Team) and C (Design Hardware).
Backward Pass—Latest Times
If the next preceding activity is a burst activity; in this case, select the smallest LS of all its immediate follow-up activities to determine its LF. The LF of activity A is controlled by activity C, which has the smallest LS of 10 days.
Determining Slack (or Float)
The critical path is the network path(s) that have the least slack in common. For example, if the EF for the project is 100 days, but the imposed LF or target date is set to 95 days, all activities on the critical path have a slack of minus 5 days.
SNAPSHOT FROM PRACTICE 6.2
Using the Forward and Backward Pass Information
Returning to the Automated Warehouse project network in Figure 6.8, what does a 35-day delay for activity F (Patient Request Development) mean to the project manager. More broadly, the project manager soon learns that time off is important because it allows flexibility in scheduling scarce project resources—personnel and equipment—to be used in more than one parallel activity or another project. Knowledge of the four activity times ES, LS, EF and LF is invaluable for the planning, scheduling and control phases of a project.
ES and LF tell the project manager the time frame in which the activity must be completed. In addition, if for some reason the project must be accelerated to meet an earlier date, it is possible to select those activities, or a combination of activities, that will cost less to shorten the project.
Level of Detail for Activities
Once the critical path is known, it is possible to tightly manage the resources for the activities on the critical path so that mistakes are not made that will result in delays. Similarly, if the critical path is delayed and time must be made up by shortening one or more activities on the critical path to offset any negative slack, it is possible to identify the activities on the critical path that cost the least to shorten. If there are other trails with very little slack, it may be necessary to shorten the activities on these trails as well.
In some cases, there may be too much information left to manage, and this can result in higher overhead costs. Managers of small projects have managed to minimize the level of detail by eliminating some of the preparatory steps for drawing networks.
Practical Considerations
Larger companies are also recognizing the costs of information overload and are working to reduce the level of detail in networks.
Network Logic Errors
Use of Computers to Develop Networks
Project management software can be a tremendous help in the hands of those who understand and are familiar with the tools and techniques discussed in this text. However, there is nothing more dangerous than someone using the software with little or no knowledge of how the software generates its output. Input errors are very common, and it takes someone skilled in the concepts, tools, and information systems to recognize that errors exist so that false actions are avoided.
Calendar Dates
Multiple Starts and Multiple Projects
Extended Network Techniques to Come Closer to Reality
Laddering
Use of Lags to Reduce Schedule Detail and Project Duration
The relationship between beginning and end represents the typical, generic networking style (used at the beginning of this chapter). Using finish-to-start, the duration of the activity can be as little as 1 day and the delay as long as 19 days. The use of delays from start to finish must be carefully monitored to ensure their validity.
A simple rule to follow is that the use of finish-to-start delays must be justified and approved by someone responsible for a major part of the project. Start-to-start relationship reduces network details and project delays by using delay relationships.
SNAPSHOT FROM PRACTICE 6.3
Concurrent Engineering basically breaks activities into smaller segments so that work can be done in parallel and the project can be expedited (Turtle, 1994). Of course, the same result can be achieved by dividing an activity into small packages that can be implemented in parallel, but this latter approach significantly increases the networking and tracking detail. For example, testing cannot be completed earlier than four days after the prototype is completed.
This relationship represents situations in which the completion of an activity depends on the start of another activity. For example, system documentation may not end until three days after testing has begun (see Figure 6.18).
An Example Using Lag Relationships—the Forward and Backward Pass
All relevant information to complete the system documentation will be produced after the first three days of testing. You can identify the critical path in Figure 6.20 by following the dotted line on the network. If there is a delay relationship, each activity should be checked to see if the start or finish is restricted.
For example, in the forward pass, the EF of activity E (test system) (17) is determined by the end of activity D (install software) and the delay of one time unit (16 + delay 1 = 17). Finally, in the backward pass, the LS of activity A (design system) is controlled by activity B (hardware ordering) and the delay relationship with activity A.
Hammock Activities
The critical path follows the start and end constraints of the activity that occur due to the use of the additional available relationships and the imposed delays. A hammock activity can be used to indicate the need for this resource and to distribute costs across this segment of the project. The hammock duration is simply the difference between the EF for the last activity and the ES for the first activity.
Many project managers feel the project network is their most valuable exercise and planning document. The use of delays can result in the start or end of an activity becoming critical.
Key Terms
In time-sensitive projects, project managers closely monitor the critical path, often assigning their best personnel to those activities. Some software simply called the entire activity critical rather than identifying the start or end as critical. Finally, hammock activities are useful in tracking the costs of resources used for a particular segment of a project.
Hammock activities can also be used to reduce the size of a project network by grouping activities together for simplicity and clarity. All discussed improvements to the original AON methodology contribute to better project planning and control.
Review Questions
SNAPSHOT FROM PRACTICE
Discussion Questions
Exercises
Tip: Start with the last activity (wedding reception) and work your way back to the beginning of the project. The project information for Air Control Company's special order project is presented here. Given the project network that follows, complete the forward and backward flow, calculate activity slack, and identify the critical path.
The clearing of the main ski slopes can begin after the area for the chair lift is cleared;. The main frame for the generator house can begin once the foundation is complete; framing will take 12 days. From the following information, calculate the early, late, and slack times for the project network.
Based on the information in the following delay exercises, calculate the early, late, and slack times for the project network.* Which activities on the critical path have only the start or finish of the activity on the critical path.
Procurement of prototype parts will only involve 2 days of work, but it will take 8 days for the parts to be delivered. Likewise, order stock components will take 2 days work and 8 days delivery, and order custom components will take 2 days work and 13 days delivery. The management is still not satisfied with the schedule and wants the project to be completed as soon as possible.
A team member noted that the network contained only end-to-end relationships and that it might be possible to reduce project duration by creating end-to-end delays.
Advantage Energy Technology Data Center Migration * — Part A
The data center supervisor requested that Brian replace all the old power supplies and data cables. Renovation of the new data center can begin as soon as contractors are hired. Brian estimates it will take five days to install the power supply and a week to run all the data cables.
After completing the power check, he has a week to install his test servers. The test servers test all primary network functions and act as security before the network is taken offline.
ASSIGNMENT
During this time it can initiate a power check to ensure that each of the racks has sufficient voltage. Batteries must be charged, ventilation installed and servers tested before management can make sure the new infrastructure is safe, which will take two days. They will then sign off on the primary systems check, taking a day of intensive meetings.
Now that the official date is set, the network will be closed for a day. Brian will do the migration over the weekend - two days - when user traffic is at a low point.
Ventura Baseball Stadium—Part A
Brian is happy that everything has gone well so far and is confident that the move will go just as smoothly. The contract includes a penalty clause of $500,000 per day of delay after April 3. Percival Young, the company's president, expressed optimism about winning the contract, revealing that the company could earn as much as $5 million from the project.
Technical Details for the Ventura Baseball Stadium
