CPM – Critical Path Method

CPM – Critical Path Method

Software Project Management 2Maria Petridou

• It is determined by adding the times for the activities in each sequence.

• CPM determines the total calendar time required for the project.• If activities outside the critical path speed up or slow down (within

limits), the total project time does not change.• The amount of time that a non-critical activity can be delayed

without delaying the project is called slack-time.

CPM – Critical Path Method

Software Project Management 3Maria Petridou

ET – Earliest node time for given activity duration and precedence relationships

LT – Latest node time assuming no delays

ES – Activity earliest start time LS – Activity latest start time EF – Activity earliest finishing time LF – Activity latest finishing time Slack Time – Maximum activity delay time

Dummy Activities

Software Project Management 4Maria Petridou

• Sometimes it is necessary to insert dummy activities (duration zero) in order to maintain the clarity of the diagram and the precedence relationships between activities.

• In activity-on-arrow PERT diagrams, each activity must be uniquely identifiable by its start and end nodes.

• However, sometimes multiple tasks have the same predecessors and successors.

Dummy Activities

Software Project Management 5Maria Petridou

Case One• A task should be uniquely identifiable from its start node and finish

node This means that two or more tasks cannot share the same start

and finish nodes

1 2 3 4A D

C Tasks B and C have the same start and finish

nodes

Task D has immediate

predecessors of B and C

B

Network Planning Techniques• Program Evaluation & Review Technique (PERT):– Developed to manage the Polaris missile project– Many tasks pushed the boundaries of science &

engineering (tasks’ duration = probabilistic)

• Critical Path Method (CPM):– Developed to coordinate maintenance projects in the

chemical industry– A complex undertaking, but individual tasks are routine

(tasks’ duration = deterministic)

PERT and CPM

• Graphically display the precedence relationships & sequence of activities

• Estimate the project’s duration• Identify critical activities that cannot be

delayed without delaying the project• Estimate the amount of slack associated with

non-critical activities

8

Project Activitiesin Constructing a House

9

PERT Network forConstructing a House