__A GUIDE TO NETWORK
ANALYSIS
by__

__MICHAEL C GLEN__

The core technique available to Project Managers for planning and controlling their projects is Network Analysis. This short guide will provide a basic understanding of networking principles before applying them to the computer.

Network Analysis or Critical Path Analysis (CPA) or the American “Program, Evaluation and Review Technique” (PERT) is one of the classic methods of planning and controlling the progress of projects.

Effective planning of projects requires careful thought and the application of logic. To illustrate this planning tool, let's consider the manufacture of a small item. Some typical processes might be:

*cutting
finishing
assembling
purchasing
machining
testing
designing*

All these processes are called
‘**ACTIVITIES**’ or ‘**TASKS’**

List **WHAT** has to be
done.

Hint: try thinking of verbs
ending in “...** ing”**, like

Do not consider at this stage
who is going to do what, concentrate on __WHAT.__

An activity or task is represented by a rectangle, thus:

Decide the **ORDER** in
which it is to be done.

Some steps are obvious: we,
perhaps, cannot ** test** until

*
designing**
purchasing cutting machining assembling testing finishing*

Writing this out as a network:

We put the tasks into rectangles and join them with arrows to show the sequence or precedence: the logical relationships between them.

Suppose that once we have
bought the materials, some need cutting to size and others
need turning on a lathe. The tasks of ** machining**
and

Let's add another task: the **
writing** of a set of test instructions. Where
would

Now let's say we need to have
our draughtsman produce some illustrations for our test
instructions. When the ** writing** and the

And so the network is built up, often cuing the mind to missing tasks.

In this step always assume you
have __infinite resources__ so that who does what does
not cloud the issue – concentrate only on the ** LOGIC**.

Having completed the network, we can begin the analysis. Firstly, we need to know the duration of each task and write it into the network. For convenience, we will write the durations in days, thus:

In this step, always reduce
the resource requirement to the duration of ** ONE**
person to give maximum flexibility for you to add further
resources later when the project begins to run late.
[However, there are rare cases when tasks cannot physically
be performed by one resource, in which case consider the
time taken for both of them working together. For example,
carrying a very long plank that requires a person at each
end, adding more resources will not necessarily reduce the
duration and might even slow it down if they get in the way
of each other, but you must have two. Checking the brake
lights on a car is another example.]

Now we can calculate how long
the project will take. We start by calculating the
shortest time or **'earliest finish'**. So if we start
at time **'0'** the earliest day that ** design
**can be finished is day

For the ** purchasing**
task, we thus say the

The earliest time both **
cutting** and

Note that the task **
machine** has some

So, the calculations show that
the whole project will take **52** days or the **
'earliest finish time'** is **52** days.

You will remember that when we
considered the task ** machine** we found that it
had some

The next part of the analysis
of the network is to find the **CRITICAL PATH**. By
definition the **Critical Path** is the shortest time
path through the network. In such a simple network, it is
easy to calculate the amount of slack available for each
task, but in a complicated network, it is not easy to 'see'
which tasks have slack and which have none.

So far we have calculated the
**Earliest Start Time (EST)** and the **Earliest Finish
Time (EFT)** for each task. The next step in calculating
the critical path is to re-time the network starting at the
end and calculate the **Latest Start Time** (**LST**)
and **Latest Finish Time (LFT**) for each task.

So, beginning with the
earliest finish time for the whole project, ie **52**
days, we subtract the time of the last task giving **49**
days as the latest time the ** finishing** task can
begin without affecting the outcome.

The latest time we can start
** testing** is

But what about ** cutting**
and

So, when making the backward pass to calculate the latest times, you have to consider all the arrows coming out of a task box and select the lowest or shortest time to that task.

Now to finish the backward
pass we calculate the latest times for ** design**
to finish and start. The latest time

Now we have completed the
timings by a forward and backward pass, we can look for
slack. In this simple network, it is easy to see that the
tasks ** machine**,

So, to find the critical path,
we look first at the tasks whose earliest and latest start
times are identical. Then we engage brain to determine
which path between such tasks has no slack. That path is
then, by definition, the **critical path**. On the
diagram below, the critical path is indicated by **bold**
lines.

Well, there we are!

The critical path has been identified, we know the total time of the project and we know how much slack there is in the non-critical tasks. We have a structured plan that is logical and ordered. All we have to do now is assign resources, put it into action and control the results!

**ãMichael
C Glen 1995**