Critical Path Mapping with Activity Network Diagrams

[Steven Bonacorsi]

The activity network diagram is a method of displaying the timelines
of all the various subtasks that are involved in any project. By doing
this, the total task duration and the earliest and latest start and
finish times for each task are also calculated and displayed. In
addition to showing which subtasks are critical to on-time task
completion, the activity network diagram can help determine where
extra effort to speed a subtask will have the greatest payoff to
overall speed.

The activity network diagram has had a relatively long history, dating
back to the 1930s. In the 1950s, the technique emerged as the Program
Evaluation Research Technique (PERT) and as the Critical Path Method
(CPM). There are several ways to represent the output of the PERT/CPM
process.

The method called the activity-on-arrow or, more simply, the arrow
diagram will be reviewed in this article. An arrow diagram treats
numbered nodes as instantaneous stop/start points for activities. The
activities themselves are considered to take place on the arrows
connecting the nodes.

What can it do for you?

An activity network diagram can show you which activities or which
series of activities is critical to the timing of a more complex
collection of interactive activities. This can be very helpful in
deciding where and when to apply extra energy to keep projects on
time. Creating an activity network diagram is time-consuming, however,
so you should consider these questions before you decide to create
one:

1. Is the task a complex one with simultaneous paths that must be
coordinated? Creating a diagram for a relatively simple task may be a
waste of time.

2. Are the durations of the subtasks known with relative certainty? If
the actual timing of events is markedly different from diagram times,
it will have little value, and people will dismiss the diagram as a
useless exercise.

3. Are the task and the timing of the task completion critical to the
organization? The effort involved in creating a diagram should be
applied to tasks that have little margin for timing error and have
either serious consequences if completion of the overall task is
delayed or large rewards if completion can be sped up.

Critical path mapping can be especially valuable in project bounding
and in the measure and improve phases of the Lean Six Sigma
methodology.

How do you do it?

1. Assemble the right team. The team must have either personal
knowledge of the timing of all the subtasks involved or connection to
that information. The team should include managers and other employees
as close to the actual situation as possible.

2. Identify all of the subtasks necessary to complete the overall
task. You might use brainstorming techniques or begin with a list of
tasks from a previous project. Record the tasks so that you can
rearrange them. A good way to do this is to write each job on the top
half of a 3x5 card or Post-It(TM) note. (The bottom half of the card
will be used for timing data later in the process.)

3. Put the activity cards in the sequence in which they must be
performed to complete the overall task. To do this, create paths or
strings of tasks that follow one after the other. These strings will
often describe sequences of activities that occur in parallel with
each other. After all the activities are in some string or path of
activities, create the overall sequence by connecting the paths. These
connections will show where jobs or tasks require input from parallel
sequences before the next task can begin. Feel free to add new cards
for missing tasks or to remove duplicates.

4. Assign time duration to each task or job. Write it on the bottom
half of the card. Since you will be adding these times, keep the
numbers consistent. For instance, do not have some cards showing days
for completion, some showing hours and some showing minutes. Select
the Lowest Common Denominator.

5. Calculate the shortest possible time within which the overall task
can be completed by adding the times of each subtask to find the path
of the longest cumulative duration. This is the critical path. Knowing
the critical path is important because this will tell if the time
objectives of the project are attainable. The critical path identifies
those jobs or tasks that have no slack. Each must be done on time if
the project is to stay on schedule. The critical path also identifies
targets for improvement to increase speed. (If tasks on the critical
path can be sped up, the overall time to complete the project may be
able to be shortened. Remember, however, that if a task on the
critical path is sped up, a different path may become the critical
path.)

6. Calculate the earliest starting and finishing times and the latest
starting and finishing times for each job or subtask in the project.
Begin at the start of the diagram. The earliest start time for each
job is the cumulative duration of all the previous jobs on that path.
The earliest finish time is the earliest start time plus the duration
of that task. Repeat this process for each job on each path until you
reach the finish point. Next calculate the latest start and finish
times. Begin with the earliest finish time at the end of the diagram.

To calculate the slack time for any job or task, subtract the earliest
start time from the latest start time. All of the jobs on the critical
path, by definition, will have zero slack time. Remember that slack
time is dependent on the time of completion of the previous job or
task. If some of the slack time in a path other than the critical path
is used in an early task, the slack times for the remaining tasks in
that path will each be reduced by that amount.)

Note: A dummy is an extra node symbol used to clarify an activity
network diagram if one node has more than one job or task feeding into
it from one other node. Since the diagram cannot show two activities
coming from one node and going to another, a dummy node is created,
with zero as the job duration shown on the arrow connecting them.

Review the completed activity network diagram with the people who will
be doing the work described by it. Consider any feedback. Expand or
modify the diagram as necessary to fit the actual situation.

Now what?

As a tool, the activity network diagram is like a time-map of any time-
sensitive project. As you are proceeding down the paths of the
project, the map will help keep you on track. If you should falter or
wander off the path, the map can be used to help get you back on the
critical path. This description of the activity network diagram will
help you to manually calculate and construct a process map.

Automated tools, such as SigmaFlow, are also available. SigmaFlow
produces other valuable scheduling information, as well. Computer
scheduling programs like SigmaFlow are able to easily deal with
complex processes, recalculating times whenever you modify the data.

A critical path map can help uncover opportunities for increasing
speed. Used in this fashion, critical path mapping is another tool to
make the Lean Six Sigma method work smoothly. It is important to
remember, however, that these diagrams, whether produced manually or
by computer, are not intended to drive you, but to signal you if
something is wrong.


Steven Bonacorsi is a Certified Lean Six Sigma Master Black Belt
instructor and coach. Steven Bonacorsi has trained hundreds of Master
Black Belts, Black Belts, Green Belts, and Project Sponsors and
Executive Leaders in Lean Six Sigma DMAIC and Design for Lean Six
Sigma process improvement methodologies. Bought to you by the Process
Excellence Network the world leader in Business Process Management
(BPM)

Author for the Process Excellence Network (PEX Network / IQPC)

Process Excellence Network
Steven Bonacorsi, President of International Standard for Lean Six
Sigma(ISLSS)
Certified Lean Six Sigma Master Black Belt
47 Seasons Lane
Londonderry, NH 03053
Phone: +(1) (603) 401-7047
E-mail: sbona...@islss.com
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