Use case points - an estimation approach

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Use Case Points

Gautam Banerjee

Use Case Points
-An Estimation Approach

Gautam Banerjee
August 2001

W hile the information in this publication is believed to be accurate, the author makes no warranty of any kind to this
material including, but not limited to, the implied warranties of merchantability and fitness for a particular purpose, the
author shall not be liable for errorscontained herein, or for incidental or consequential damages in connection with the
furnishing, performance or use of this material.
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1

Use Case Points

Gautam Banerjee

Table of ContentsIntroduction.....................................................................................................................3
Background.....................................................................................................................3
Use Case Points Based Estimation...................................................................................3
Classifying Actors and UseCases................................................................................4
Technical and Environmental Factors ..........................................................................5
Producing Estimates ....................................................................................................6

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Use Case Points

Gautam Banerjee

Introduction

Estimates of cost and schedule insoftware projects are based on a prediction of the size of the
future system. Unfortunately, the software profession is notoriously inaccurate when estimating
cost and schedule. Preliminary estimates of effort always include many elements of insecurity.
Reliable early estimates are difficult to obtain because of the lack of detailed information about
the future system at an early stage. However,early estimates are required when bidding for a
contract or determining whether a project is feasible in the terms of a cost-benefit analysis. Since
Process prediction guides decision-making, a prediction is useful only if it is reasonably accurate.
Traditional cost models take software size as an input parameter, and then apply a set of
adjustment factors or 'cost drivers' to compute anestimate of total effort. In object-oriented
software production, use cases describe functional requirements. The use case model may
therefore be used to predict the size of the future software system at an early development stage.
This paper describes a simple approach to software cost estimation based on use case models:
the 'Use Case Points Method'. The method is not new, but has not becomepopular although it is
easy to learn. Reliable estimates can be calculated in a short time with the aid of a spreadsheet.

Background

Cost models like COCOMO and sizing methods like Function Point Analysis (FPA) are well
known and in widespread use in software engineering. But these approaches have some serious
limitations. Counting function points requires experts.
In 1993 the 'Use CasePoints' method for sizing and estimating projects developed with the
object-oriented method was developed by Gustav Karner of Objectory (now Rational Software).
The method is an extension of Function Point Analysis and Mk II Function Point Analysis (an
adaption of FPA mainly used in the UK), and is based on the same philosophy as these methods.
A few cost estimation tools apply use case pointcount as an estimation of size, adapting Karner's
method. Karner's work on Use Case Point metrics was written as a diploma thesis at the
University of Linköping. It was based on just a few small projects, so more research is needed to
establish the general usefulness of the method. The work is now copyright of Rational Software,
and is hard to obtain.

Use Case Points Based Estimation

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