Category: PERT & CPM

What is Crash Cost?

What is Crash Cost?

Crash cost of doing any job is more than the normal cost. The cost of doing a job is minimum when normal time is allotted to the job. When a program crashes i.e., less than the normal time is allotted, then all or some of the activities have to be speeded up. Speeding up of a job involves extra cost. In other words, the cost of doing a job is minimum when it is done at normal speed. But when it is speeded up it involves extra resources and thus becomes costly which is known as crash cost.

What is a Dummy Activity? Example, Purpose & Rules

What is a Dummy Activity?

• A dummy activity is an imaginary activity.
• It is represented by the dashed arrow.
• Does not consume any time and resources.

Purpose of Dummy Activity

There are two reasons to use dummy activity one is grammatical purpose another one is logical purpose.

1. Grammatical Purpose

We will try to understand it by seeing the picture. The activity ‘A’  starts from node 1, and B also starts from node 1, and both activities end at node 2.

We can represent it as follows:

• A ⇒(1,2)
• B ⇒(1,2)

Now you can see that both identities are the same, so they lose their identity. Here, the main logic is that they both start at the same time and also finish at the same time. To find out their identity with logic, we used a dummy activity as fig.2

Now we can represent like as
A ⇒(1, 3)
B ⇒(1, 2)

2. Logical Purpose

You can not provide the same activity twice in your network diagram.

In the above picture, it shows that the P activity is two times in a single network diagram. So to avoid these types of errors in the network diagram, we provide a dummy activity.

Rules for Removing Redundant Dummy Activity

You can use as many dummy as you wish, but sometimes it asks to remove dummy which is not required. Five golden rules are used for removing redundant dummy.

Rule: 1

If the dummy originated from one node.

Here, the P1 dummy is not required

Rule: 2

If a dummy is the only arrow going into a node.

Here, A3 dummy is not required

Rule: 3

If two activities have set of a predecessor.

Rule: 4

If two activities have the same set of a successor.

Here, A2 and B2 dummy activities are not required

Rule: 5

Do not repeat already implied logic.