Understand how scheduling tools works-Draw network diagram yourself

All planning engineers and most project managers can create a Gantt Chart (Bar Chart) using different scheduling tools such as Primavera and Microsoft Project. However, only a few of them know how the scheduling software calculates the network diagram to estimate the project duration, project floats, and critical path.

In this topic, we will discuss how any software calculates the network diagram to estimate project duration, float, and the critical path. Using the following figure of the network diagram we will see how the calculation works.

Each circle represents a task, and each line between represents the relationship. For example, task S needs to be finished before task A can start. Moreover, tasks B and C should be finished before task F can start, and so on.

First of all, you need to draw a rectangular for each task as per the following figure.

ES = Early start

EF= Early finish

ID = task ID

Slack= Total Float

D= Task duration

LS= Late start

LF= Late finish

If we assumed the duration (D) for each activity as per the following:

Task	Task Duration E
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B	11
C	12
D	9
E	2
F	10
G	3
H	2
I	6
J	8

To calculate the values for each rectangular (task), you should first fill in the duration (E), and calculate Early Start (ES) and Early Finish (EF).

This process is called Forward Calculation EF = (ES + D) – 1

After you finish all the tasks, from the last one you need to go back and calculate the Late Finish (LF), Late Start (LS), and Total Float (Slack). This process is called Backward Calculation LS = (LF – D) + 1

You should have a shape that is close to the following figure

The Green color represents the Forward Calculation EF = (ES + D) – 1, and the yellow color represents the Backward Calculation LS = (LF – D) + 1

Try to solve this network diagram by yourself, this will help you to develop common sense when you are dealing with scheduling software such as Primavera or Microsoft Project. When you finish, you will be able to find the project’s total duration, which will be founded on task J Early Finish or Late Finish, and you will be able to define the critical Path.

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