The Constructive Cost Model (COCOMO) is an algorithmic software cost estimation model developed by Barry W. Boehm. The model uses a basic regression formula with parameters that are derived from historical project data and current as well as future project characteristics.
First published in Boehm's 1981 book Software Engineering Economics as a model for estimating effort, cost, and schedule for software projects. It drew on a study of 63 projects at TRW Aerospace where Boehm was Director of Software Research and Technology. The study examined projects ranging in size from 2,000 to 100,000 lines of code, and programming languages ranging from assembly to PL/I. These projects were based on the waterfall model of software development which was the prevalent software development process in 1981.
References to this model typically call it COCOMO 81. In 1995 COCOMO II was developed and finally published in 2000 in the book Software Cost Estimation with COCOMO II. COCOMO II is the successor of COCOMO 81 and is better suited for estimating modern software development projects. It provides more support for modern software development processes and an updated project database. The need for the new model came as software development technology moved from mainframe and overnight batch processing to desktop development, code reusability, and the use of off-the-shelf software components. This article refers to COCOMO 81.
COCOMO consists of a hierarchy of three increasingly detailed and accurate forms. The first level, Basic COCOMO is good for quick, early, rough order of magnitude estimates of software costs, but its accuracy is limited due to its lack of factors to account for difference in project attributes (Cost Drivers). Intermediate COCOMO takes these Cost Drivers into account and Detailed COCOMO additionally accounts for the influence of individual project phases.
COCOMO applies to three classes of software projects:
- Organic projects - "small" teams with "good" experience working with "less than rigid" requirements
- Semi-detached projects - "medium" teams with mixed experience working with a mix of rigid and less than rigid requirements
- Embedded projects - developed within a set of "tight" constraints. It is also combination of organic and semi-detached projects.(hardware, software, operational, ...)
The basic COCOMO equations take the form
- Effort Applied (E) = ab(KLOC)bb [ person-months ]
- Development Time (D) = cb(Effort Applied)db [months]
- People required (P) = Effort Applied / Development Time [count]
where, KLOC is the estimated number of delivered lines (expressed in thousands ) of code for project. The coefficients ab, bb, cb and db are given in the following table:
Basic COCOMO is good for quick estimate of software costs. However it does not account for differences in hardware constraints, personnel quality and experience, use of modern tools and techniques, and so on.
Intermediate COCOMO computes software development effort as function of program size and a set of "cost drivers" that include subjective assessment of product, hardware, personnel and project attributes. This extension considers a set of four "cost drivers",each with a number of subsidiary attributes:-
- Product attributes
- Required software reliability
- Size of application database
- Complexity of the product
- Hardware attributes
- Run-time performance constraints
- Memory constraints
- Volatility of the virtual machine environment
- Required turnabout time
- Personnel attributes
- Analyst capability
- Software engineering capability
- Applications experience
- Virtual machine experience
- Programming language experience
- Project attributes
- Use of software tools
- Application of software engineering methods
- Required development schedule
Each of the 15 attributes receives a rating on a six-point scale that ranges from "very low" to "extra high" (in importance or value). An effort multiplier from the table below applies to the rating. The product of all effort multipliers results in an effort adjustment factor (EAF). Typical values for EAF range from 0.9 to 1.4.
|Very Low||Low||Nominal||High||Very High||Extra High|
|Required software reliability||0.75||0.88||1.00||1.15||1.40|
|Size of application database||0.94||1.00||1.08||1.16|
|Complexity of the product||0.70||0.85||1.00||1.15||1.30||1.65|
|Run-time performance constraints||1.00||1.11||1.30||1.66|
|Volatility of the virtual machine environment||0.87||1.00||1.15||1.30|
|Required turnabout time||0.87||1.00||1.07||1.15|
|Software engineer capability||1.42||1.17||1.00||0.86||0.70|
|Virtual machine experience||1.21||1.10||1.00||0.90|
|Programming language experience||1.14||1.07||1.00||0.95|
|Application of software engineering methods||1.24||1.10||1.00||0.91||0.82|
|Use of software tools||1.24||1.10||1.00||0.91||0.83|
|Required development schedule||1.23||1.08||1.00||1.04||1.10|
The Intermediate Cocomo formula now takes the form:
where E is the effort applied in person-months, KLoC is the estimated number of thousands of delivered lines of code for the project, and EAF is the factor calculated above. The coefficient ai and the exponent bi are given in the next table.
Software project ai bi Organic 3.2 1.05 Semi-detached 3.0 1.12 Embedded 2.8 1.20
The Development time D calculation uses E in the same way as in the Basic COCOMO.
Detailed COCOMO incorporates all characteristics of the intermediate version with an assessment of the cost driver's impact on each step (analysis, design, etc.) of the software engineering process.
The detailed model uses different effort multipliers for each cost driver attribute. These Phase Sensitive effort multipliers are each to determine the amount of effort required to complete each phase. In detailed cocomo,the whole software is divided in different modules and then we apply COCOMO in different modules to estimate effort and then sum the effort
In detailed COCOMO, the effort is calculated as function of program size and a set of cost drivers given according to each phase of software life cycle.
A Detailed project schedule is never static.
The five phases of detailed COCOMO are:-
- plan and requirement.
- system design.
- detailed design.
- module code and test.
- integration and test.
- Estimation in software engineering
- Comparison of development estimation software
- Software engineering economics
- Cost overrun
- Putnam model
- Barry Boehm. Software Engineering Economics. Englewood Cliffs, NJ:Prentice-Hall, 1981. ISBN 0-13-822122-7
- Barry Boehm, Chris Abts, A. Winsor Brown, Sunita Chulani, Bradford K. Clark, Ellis Horowitz, Ray Madachy, Donald J. Reifer, and Bert Steece. Software Cost Estimation with COCOMO II (with CD-ROM). Englewood Cliffs, NJ:Prentice-Hall, 2000. ISBN 0-13-026692-2
- Kemerer, Chris F. (May 1987). "An Empirical Validation of Software Cost Estimation Models". Communications of the ACM 30 (5): 416–42. doi:10.1145/22899.22906.
- COCOMO II website
- COCOMO 81 Calculator
- PHP source code for COCOMO II Software cost estimation calculator on GitHub