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Critical path method

Critical path method

Critical Path : is a combination of activities that, if any are delayed, will delay the project’s finish date.

Purpose of critical path method:

1. To calculate the project’s finish date.

2. To identify how much individual activities in schedule can slip with delaying project

3. To identify the activities with the highest risk that cannot slip without changing the project finish date.

Float Calculation using CPM

● Float is the amount of time an activity can be delayed without delaying the project end date.

● Float is always Zero on Critical Path activities.

● Critical Path is longest path of network diagram.

Each activity have same box

ES : Early Start

EF : Early Finish

LS: Late Start

LF : Late Finish

Float = LS-ES or LF-EF

Float Calculation

Late Start(LS) – Early Start(ES)

OR

Late Finish(LF) – Early Finish (EF)

Example

You are the project manager of project at SkillsMag and following activities are identified along with the activity effort.

Project A
Activity	Preceding  Activity	Duration (Wks)
Start	None	0
A	Start	1
B	Start	3
C	Start	5
D	A	9
E	B,C	2
F	C	3
G	D	4
H	E	8
I	F	2
End	G,H,I	0

First  prepare schedule network Diagram

Step 2:

Calculate the ES and EF of each activity using forward pass technique.

Remember the following formulas for calculation.

Early Start (ES) = Early Finish(EF) of Predecessor activity. If no predecessor activity exists then 0.

Early Finish (EF) = Early Start (ES) + Activity Duration

Activity E is highlighted as it has 2 predecessor activities. we take higher one.

Step 3:

Under this technique, we will start calculating the value from last activities on the path i.e. from G,H and I.

Thumb Rule : Take highest EF.

Our case we have EF 15 is highest for activity H.

The formula for LS(Late start) is

Late Start (LS) = Late Finish – Activity Duration

Activity c has 2 successor activity,take lowest one.

Step 4 : Float calculation