Figure 4-3 EXAMPLES OF NETWORK GRAPHIC CONVENTIONS
Figure 4-4 CONFERENCE PLANNING NETWORK
start -
Identify Critical Activities 37 pleted on schedule. The time to complete the project is the longest time path through the network. The sequence of activities that makes up the longest path is known as the critical path. As long as activities on the critical path are going well, the project will remain on schedule. If, however, any of these activities takes longer than originally estimated, the project completion time will be extended. Of course, the manager will try to obtain additional resources for critical path activities to restore the project to its original completion date. Alternatively, when additional resources are available (say, from project activities that are not on the critical path), the manager may be able to reallocate them to critical path activities and get the project back on schedule.
ACTIVITY START AND COMPLETION TIMES
Now let us define the four calculated values (ES, LS, EF, LF, see Figure 4-2) associated with each activity node. These calculated values will be used to deter- mine the project completion time and the critical path.
Earliest Start and Earliest Finish Times
The earliest start (ES) time for an activity is the earliest time at which all predeces- sor activities of that activity have been completed and the subject activity can begin. The ES time of an activity having no predecessor activities is arbitrarily set to 0. The EF time of an activity is equal to its ES time plus its estimated completion time. The ES time of an activity that has one predecessor activity is the EF time of the predecessor activity. The ES time of activities having two or more predecessor activities is the maximum of the EF times of the predecessor activities. The ES and EF times for the Conference Planning Project are shown in the upper left and right corners of each activity node (see Figure 4-5).
Latest Start and Latest Finish Times
The latest start (LS) and latest finish (LF) times of an activity are the latest times at which an activity can start (LS) or be completed (LF) without increasing project completion time. To calculate these times we will work backward in the network.
First set the LF time of the last activity on the network to the EF time of that activity.
The LS time of that activity is equal to its LF time minus its estimated completion time. The LF time of all the immediate predecessor activities is the minimum of the LS times of all activities for which it is the predecessor. The LS and LF times for the Conference Planning Project are shown in the lower left and right comers of each activity node (see Figure 4-5). The last numeric value in the node is the average
Figure 4-5 CONFERENCE PLANNING NETWORK SHOWING ES, LS, EF, AND LF TIMES FOR ALL ACTIVITIES
activity completion time (E) that is computed from the formula given in Chapter Three and repeated here:
LOCATING
THE
CRITICAL PATHThere are two ways to find the critical path in a network. The first way is simply to identify every possible sequence of activities through the network and compute the time to complete each of these paths. The sequence with the longest estimated completion time will be the critical path. This will work for smaller projects or for projects whose activities have mostly single predecessors. Figure 4-6 shows the calculation of all sequences of activities for the Conference Planning Project. Note that the sequence B-D-E-G-I-J-K, with a duration of 31 weeks, is the longest time path through the network and hence is the critical path.
For most projects this enumeration method is not feasible and we have to resort to the second method of finding the critical path. To do so we need to compute another quantity known as the activity slack time. Slack time is the amount of delay that could be tolerated in the starting time or completion time of an activity without causing a delay in the completion of the project. Slack time is mathematically the difference LS-ES (or equivalently, LF-EF). The sequence of
Identify Critical Activities 39 activities having zero slack is defined as the critical path. Figure 4-7 highlights the critical path for the Conference Planning Project. Also in this figure, note the slack time placed above each activity node.
Figure 4-6 ALL POSSIBLE ACTIVITY SEQUENCES
PATH DURATION
A - C - E - G - I - J - K 27 weeks A - C - F - G - I - J - K 23 weeks
B - D - E - G - I - J - K 31 weeks <-- Critical Path 6 - D - F - G - I - J - K 27 weeks
B - D - H - J - K 18 weeks
Figure 4-7 CRITICAL PATH AND SLACK TIME FOR THE CONFERENCE PLANNING NETWORK
4 4
0 2 2 7 11 20
- A 2 - C 5 - , E 9 - 4 6 6 1 1 11 20
20 22 22 28
-
- G 2 9 1 6 -20 22 22 28
4 28 29 29 31
11 16
L J
- F 5 - 28 29 29 31
15 20
13
0 5 5 11, 11 15
- B 5 - D 6 - H 4 -
10 5 1 1 5 111 124 2a1
USE OF THE NETWORK AND CRITICAL PATH
The project network is a tool that the project manager and project team will use throughout the life of the project. For the project manager it is a tool for planning, implementation, and control. For the project team it is a tool to brief new team members on the project and its status.
Planning
Since the network shows the interdependencies between all project activities, it allows the manager to view the project in its entirety. This bird's-eye view of the project provides the manager with a tool for scheduling activity start times, assigning project team members to activities, and reallocating resources as the project com- mences. It is inevitable that the project will vary from the planned schedule, and you will need to move resources from activity to activity and make other adjustments to maintain the original completion date.
Implementation
As the project progresses, the network can be updated to reflect what has actually happened. Some activities will be completed in less time than originally estimated. This will give the project manager additional scheduling flexibility and the opportunity to reallocate resources to perhaps bring the project in earlier or for less cost. On the other hand, some activities will require more time to complete than was originally estimated.
In these cases the project manager will first try to reallocate resources from non-critical path activities. If that doesn't provide enough additional resources, then a request for additional resources to get the project back on schedule will have to be made.
Control
By comparing the planned schedule of activities with what actually happened, the project manager will be aware of variances from plan and the appropriate corrective measures to take. In Chapter Nine we will examine this in more detail and provide additional reporting tools for analyzing and controlling project progress.
The Critical Path
The expected project duration time determined from the critical path analysis and the contracted project completion date provide useful pieces of information for making management decisions. For example:
The scheduled start and completion date for each activity.
Identify Critical Activities 41 The activities whose completion must occur exactly as scheduled in order for the project to be kept on schedule.
The amount of delay that can be tolerated in non-critical path activities without causing a delay in the scheduled project completion date.
Activities whose resources might be diverted to critical path activities if the need arises.
PROJECT NETWORK QUALITY CONTROL CHECKLIST
YES OR NO
- 1. The first node or activity is "start" at the left-hand side of the network; the last node is "end" at the far right-hand side.
- 2. The logic of sequencing activity nodes from left to right was done by asking,
"What is the next activity that can begin only after the preceding activity is completed?"
- 3. "~&ne" was disregarded in this exercise; you are only concerned with which activity precedes and succeeds another activity.
- 4. No activity node should overlap any other. If so, make two discrete activities from that one.
5 . All activity nodes flow from left to right in a linear way with no feedback loops.
6 . All activity nodes are connected by a line on the network; no node is left unconnected.
- 7. All activities fit logically into the network; if not, the activity should be elimi- nated or re-created.
- 8. Each path can be easily traced from the start to the end node.
- 9. Parallel activity nodes on parallel paths can be drawn if work (activities) can be done simultaneously.
PROJECT NETWORK ACID TEST
1. An outsider could easily understand the sequencing of this project by studying your network diagram.
2. Each path can be traced with ease.
3. It is obvious which activities can be worked on simultaneously.
4. All activities are sequenced logically.
5. You are ready to pass this network up for approval.
6. Your core project team reviewed and approved the network.
5-PHASE PROJECT MANAGEMENT
b-PLANNING A, IMPLEMENTATION
,IDEFINE State the Problem
ldentify Project Goals List the Objectives
Determine Preliminary Resources ldentify Assumptions
and Risks
2