5th Edition PMBOK® Guide–Chapter 8: Activity Network Diagram


1.  Introduction

This is the sixth in the series of posts on 7 different quality management tools that can be used in conjunction with process 8.2 Perform Quality Assurance.   This last tool is that of activity network diagrams.   In connection with project management, they are mostly associated with project scheduling.

If you are asked:  how long will this project take, you need to create an activity network diagram. Once you create the diagram you need to figure out the critical path, which is the path from the start to the finish of the project with the longest cumulative duration.  Those activities on the critical path may not be delayed without delaying the schedule as a whole. Other activities that are NOT on the critical path may be delayed for a certain period of time without affecting the schedule, and the amount of “wiggle room” in the schedule for any given activity is called the float of that activity.

2.  Creating an activity network diagram

First step is creating the activity network diagram.

Step Description
1. Define Activities Take the work breakdown structure or WBS, which takes the general objectives of the project and breaks them down into deliverables. Then take each deliverable and list all activities it will require to accomplish it.
2. Sequence Activities Sequence the activities based on the precedence relationship between them. Some activities have to be completed before others are started, for example. Other activities may be able to be done simultaneously. Based on these relationships between activities, create a network diagram that looks like a flowchart with a box for each activity.
3. Estimate Activity Durations Add a duration to each of the boxes containing the activities.
4. Critical Path Method Calculate the duration of the various “branches” of the network in order to determine which branch is the critical path of the network.
5. Calculate “Float” or “Slack” Using the forward pass and backward pass method, calculate the total float or slack of each of the activities. NOTE: An activity on the critical path will have ZERO float BY DEFINITION

2. Critical path method

To determine how long a project will take, you need to find out the critical path, that is, the sequence of activities in the network diagram that is the longest. Other paths along the network will yield sequences of activities that are shorter than the critical path, and they are shorter by an amount equal to the float. This means that activities that have float could be delayed by a certain amount without affecting the schedule. Activities along the critical path have a float of zero. This means that any delay along the critical path will affect the schedule.

Here’s an outline of the critical path methodology.

a. You create a network diagram of all the activities.

b. You label each activity with the duration derived from process 6.4 Estimate Activity Durations.

c. You do a forward pass to determine the early start and early finish date of all activities, from the start of the project to the end of the project.

d. Once at the end of the project, you do a backward pass to determine the late start and late finish date of all activities, from the end of the project to the start of the project.

e. For each activity, you use the results of c and d to calculate the float of each activity.

f. All activities that have 0 float are on the critical path for that project.

3.  Critical path method–alternative

In practice, an easier way to get the float of each activity is to do the following:

a. You create a network diagram of all the activities.

b. You label each activity with the duration derived from process 6.4 Estimate Activity Durations.

c.  List the various paths in the network.

d.  Find out which path has the longest duration:  that is the critical path.

e.  All activities on the critical path have 0 float by definition.   Label all those activities with 0 float.

f.   Find the path with the next-highest duration compared to the critical path.    Calculate the difference between that duration and the duration of the critical path:   that is the float for any activity that has NOT already been labeled as having 0 float (in step e).

g.  Keep finding the path with the duration that is next-highest, calculate the difference between that duration and the duration of the critical path:   that is the float for any activity that has NOT already been labeled with a float in previous steps.

This method is faster than using the forward and backward pass method, which is important on the PMP or CAPM certification exam when time is your main constraint.

 

The final post tomorrow will deal with the seventh quality management tool used in conjunction with process 8.2 Perform Quality Assurance, that of matrix diagrams.

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