PMP®:CAPM® - Activity Network Diagram

What is Activity Network Diagram?

Activity network diagram is the output of Sequence activities process in Time management knowledge area. It shows the dependencies between activities.  

How it's created?

Activity network diagram is created using the Precedence Diagramming Method (PDM) which is also called as Activity On Node (AON). In the below network diagram, box or node represent activities and arrows represent dependencies.

A sample Network Diagram

Things to know in a Network Diagram

Duration

The time period it takes to complete the activity.In the above network diagram activity Y takes 5 days to complete.

Early Start (ES) , Early Finish (EF), Late Start(LS), Late Finish(LF)

In the above network diagram, both the activities can be completed within a week. If Y starts on Monday, it will be completed on Friday. If Z starts on Monday, it will be completed on Tuesday. Z is a parallel activity to Y and it can by delayed by 3 days. This is called a float or slack. If Z starts on Thursday, it will be completed on Friday.So the Early Start and Early Finish of Y are Monday and Tuesday and the Late Start and Late Finish of Y are Thursday and Friday.

There is no float for activity Y since it can't be delayed. Any delay in Y will affect the project end date. So ES,EF of Y are Monday and Friday and LS and LF of Y are Monday and Friday.

Float

There are different types of float. Free float, total float, project float and negative float.

The Finish Start relationship exists between the activities of the above network diagram.

In the above network diagram, if A is done on Monday, B will be done on Tuesday to Thursday and E will be done on Friday. C and D have float of 1 day. They can be done Tuesday and Wednesday or Wednesday and Thursday. 

Total float

The amount of time an activity can be delayed without delaying the project end date. A, B and E cannot be delayed. C and D can be delayed by 1 day. The total float of C and D is 1 day. 

Total float = LS - ES or LF - EF. In the above example, total float of C = 2-1=3-2=1.  

Total float is for the path. If C uses the total float and delayed by 1 day, then D will not have total float. 

Free float

The amount of time an activity can be delayed without delaying the early start of the follow-on (next) activity. Free float can lesser than or equal to Total float.

Free float of an activity = ES of next activity and EF of that activity. (ES of next activity - in case of many next activities, choose min of ES of next or successor activities.)

Free float of C = 2-2=0. Free float of D = 4-3=1. 

Crtical path

The longest path in the network diagram which gives the shortest duration to complete the project, in which all the activities have zero float.

There are 2 paths in the above network diagram. ABE (total duration=1+3+1=5 days) and ACDE (total duration=1+1+1+1=4 days). The critical path is ABE, since it is the longest path. This project can be completed in 5 days.

We will discuss the Critical path in the next post. 

Project float

The amount of time the project can be delayed without delaying the client imposed milestone. In the above example (project schedule 1 ) , the project float is 5 days which means the project can be delayed by 5 days. 

Negative float

The amount of time the project schedule is beyond the client imposed milestone.It is called 'negative' float since in this case the project completion need to be advanced not delayed. In the above example, (project schedule 2), the project schedule has crossed the client imposed milestone by 5 days which means it has the negative float of 5 days. 

Read next:
Critical Path Method (CPM)