Introduction to Network Analysis of Projects and CPM
The Critical Path Method (CPM) is one of several related techniques for doing project planning. CPM is for projects that are made up of a number of individual "activities." If some of the activities require other activities to finish before they can start, then the project becomes a complex web of activities. CPM can help you figure out:- how long your complex project will take to complete
- which activities are "critical," meaning that they have to be done on time or else the whole project will take longer
- whether you should try to speed up the project, and, if so,
- what is the least costly way to speed up the project.
Activities
An activity is a specific task. It gets something done. An activity can have these properties:- names of any other activities that have to be completed before this one can start
- a projected normal time duration
- a cost to complete
- a shorter time to complete on a crash basis
- the higher cost of completing it on a crash basis
CPM Analysis:
This document describes the steps for doing CPM analysis for this course. The steps will be illustrated by two examples. I recommend that you work through the examples, so that you can follow the steps yourself when you do the homework.Example 2 is especially valuable for you to work through. Excel has bugs that vary from version to version. By working through Example 2, and comparing what you get with what I got, you can find out which bugs apply to you and how to work around them when you do the assignment.
Step 1: List the activities
CPM analysis starts when you have a table showing each activity in your project. For each activity, you need to know which other activities must be done before it starts, and how long the activity takes.
Here's the example:
| Activity | Description | Required Predecessor | Duration |
| A | Product design | (None) | 5 months |
| B | Market research | (None) | 1 |
| C | Production analysis | A | 2 |
| D | Product model | A | 3 |
| E | Sales brochure | A | 2 |
| F | Cost analysis | C | 3 |
| G | Product testing | D | 4 |
| H | Sales training | B, E | 2 |
| I | Pricing | H | 1 |
| J | Project report | F, G, I | 1 |
Step 2: Draw the diagram
Draw by hand a network diagram of the project that shows which activities follow which other ones. This can be tricky. The analysis method we'll be using requires an "activity-on-arc" (AOA) diagram. An AOA diagram has numbered "nodes" that represent stages of project completion. You make up the nodes' numbers as you construct the diagram. You connect the nodes with arrows or "arcs" that represent the activities that are listed in the above table.
Some conventions about how to draw these diagrams:
- All activities with no predecessor come off of node 1.
- All activities with no successor point to the last node, which has to have highest node number.
Step 3: Set up the CPM spreadsheet
Start up a new blank spreadsheet. If you are viewing this document on the web, minimize your browser window and then start Excel. That way you can switch from one to the other by pressing Alt+Tab.
In a blank spreadsheet, type the word "Activities" in cell A1. In row 2, type the names of the activities, or their letters. (To make my spreadsheet screen shots fit better on these pages, I set the column widths to 4. You do not have to do this.)
In row 3, type "Nodes". In row 4, type in each activity's start node -- where the tail of its arrow is. Below that, in row 5, type each activity's end node -- where the head of its arrow is. Do this carefully. Mistakes here mess up everything that follows.
To the right, in K2 and K3, type the words "Start" and "End" to label those rows.
For More Information : http://hspm.sph.sc.edu/courses/j716/cpm/cpm.html
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