A Guide to Network Analysis

A GUIDE TO NETWORK ANALYSIS

by

MICHAEL C GLEN

Introduction

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 ÐŽ§Program, Evaluation and Review TechniqueЎЁ (PERT) is one of the classic methods of planning and controlling the progress of projects.

Tasks or Activities

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 ЎҐACTIVITIESÐŽ¦ or ЎҐTASKSÐŽ¦

Step 1:

List WHAT has to be done.

Hint: try thinking of verbs ending in ÐŽ§...ingЎЁ, like machining or testing.

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:

Step 2:

Decide the ORDER in which it is to be done.

Some steps are obvious: we, perhaps, cannot test until assembly has been completed, which cannot be done until the various parts have been made. So we have a logical relationship between the start of one task and the beginning of the next. We could order our list of tasks thus:

designing purchasing cutting machining assembling testing finishing

Logic Network or PERT Chart

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 cutting could run in parallel rather than consecutively, assuming we have the appropriate resources. But let's add a bit more. Say the cut parts need to be welded together before assembling ÐŽX like this:

Let's add another task: the writing of a set of test instructions. Where would writing fit in? Well, the writing cannot really start until the design is finished, though it could be carried out at the same time as the fabrication, but it must be ready before the testing can begin. Applying such logic to the relationships, we can add the task writing like this:

Now let's say we need to have our draughtsman produce some illustrations for our test instructions. When the writing and the drawing are finished, we will then need to edit the whole:

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 ÐŽV concentrate only on the LOGIC.

Time Analysis

Duration

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.]

The Forward Pass

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 10. Purchasing therefore cannot start until day 10: ie the earliest we can start purchasing is day 10, and thus the earliest finish for purchasing is 10 + 21 = day 31. We write the earliest start time in the top left-hand corner of the task box, and the earliest finish time over the right-hand corner of the task box.

For the purchasing task, we thus say the Earliest Start Time (EST) is 10 and the Earliest Finish Time (EFT) is 31

The earliest time both cutting and machining can start is day 31. So the earliest finish for cutting will be day 36 and for machining day 39. However, assembly cannot start at day 39 because the welding has not yet been done. The earliest welding can start is after the cutting is finished, ie day 36. So the earliest time assembly can start