The Constructive Cost Model (COCOMO) is an algorithmic software cost estimation model

Estimation in software engineering

The ability to accurately estimate the time and/or cost taken for a project to come in to its successful conclusion is a serious problem for software engineers. The use of a repeatable, clearly defined and well understood software development process has, in recent years, shown itself to be the...

 developed by Barry W. Boehm

Barry Boehm

Barry W. Boehm is an American software engineer, TRW Emeritus Professor of Software Engineering at the Computer Science Department of the University of Southern California, and known for his many contributions to software engineering.- Biography :...

. The model uses a basic regression

Regression analysis

In statistics, regression analysis includes many techniques for modeling and analyzing several variables, when the focus is on the relationship between a dependent variable and one or more independent variables...

 formula with parameters that are derived from historical project data and current project characteristics.

COCOMO was 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

TRW

TRW Inc. was an American corporation involved in a variety of businesses, mainly aerospace, automotive, and credit reporting. It was a pioneer in multiple fields including electronic components, integrated circuits, computers, software and systems engineering. TRW built many spacecraft,...

 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

Assembly language

An assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and other programmable devices. It implements a symbolic representation of the machine codes and other constants needed to program a given CPU architecture...

 to PL/I

PL/I

PL/I is a procedural, imperative computer programming language designed for scientific, engineering, business and systems programming applications...

. These projects were based on the waterfall model

Waterfall model

The waterfall model is a sequential design process, often used in software development processes, in which progress is seen as flowing steadily downwards through the phases of Conception, Initiation, Analysis, Design, Construction, Testing, Production/Implementation and Maintenance.The waterfall...

 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 process

Software development process

A software development process, also known as a software development life cycle , is a structure imposed on the development of a software product. Similar terms include software life cycle and software process. It is often considered a subset of systems development life cycle...

es 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.

Basic COCOMO

Basic COCOMO computes software development effort (and cost) as a function of program size. Program size is expressed in estimated thousands of source lines of code (SLOC

Source lines of code

Source lines of code is a software metric used to measure the size of a software program by counting the number of lines in the text of the program's source code...

)

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) = a_b(SLOC)^b_b [ man-months ]

Development Time (D) = c_b(Effort Applied)^d_b [months]

People required (P) = Effort Applied / Development Time [count]

where,
SLOC is the estimated number of delivered lines (expressed in thousands ) of code for project,
The coefficients a_b, b_b, c_b and d_b are given in the following table.

Software project	ab	bb	cb	db
Organic	2.4	1.05	2.5	0.38
Semi-detached	3.0	1.12	2.5	0.35
Embedded	3.6	1.20	2.5	0.32

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 COCOMOs

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.

Cost Drivers	Ratings
	Very Low	Low	Nominal	High	Very High	Extra High
Product attributes
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
Hardware attributes
Run-time performance constraints			1.00	1.11	1.30	1.66
Memory constraints			1.00	1.06	1.21	1.56
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
Personnel attributes
Analyst capability	1.46	1.19	1.00	0.86	0.71
Applications experience	1.29	1.13	1.00	0.91	0.82
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
Project attributes
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:

E=a_i(SLoC)^(b_i).EAF

where E is the effort applied in person-months, SLoC is the estimated number of thousands of delivered lines of code for the project, and EAF is the factor calculated above. The coefficient a_i and the exponent b_i 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

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 efforts multipliers for each cost drivers attribute these Phase Sensitive effort multipliers are each to determine the amount of effort required to complete each phase.

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.

The five phases of detailed COCOMO are:-

• plan and requirement.
• system design.
• detailed design.
• module code and test.
• integration and test.

See also

• Estimation in software engineering
  Estimation in software engineering
  The ability to accurately estimate the time and/or cost taken for a project to come in to its successful conclusion is a serious problem for software engineers. The use of a repeatable, clearly defined and well understood software development process has, in recent years, shown itself to be the...
  
  
• Comparison of development estimation software
  Comparison of development estimation software
  A comparison of notable Software development effort estimation software.-See also:* Software Sizing* Software metric* Software development effort estimation* Software parametric models* Cost estimation models...
  
  
• COSYSMO
  Cosysmo
  The Constructive Systems Engineering Cost Model was created by Ricardo Valerdi while at the University of Southern California Center for Software Engineering. It gives an estimate of the number of person-months it will take to staff systems engineering resources on hardware and software projects...
  
  
• Software engineering economics
• Cost overrun
  Cost overrun
  A cost overrun, also known as a cost increase or budget overrun, is an unexpected cost incurred in excess of a budgeted amount due to an under-estimation of the actual cost during budgeting...
  
  
• Putnam model
  Putnam model
  The Putnam model is an empirical software effort estimation model.The original paper by Lawrence H. Putnam published in 1978 is seen as pioneering work in the field software process modelling....
  
  
• SEER-SEM
  SEER-SEM
  SEER for Software is an algorithmic project management software application designed specifically to estimate, plan and monitor the effort and resources required for any type of software development and/or maintenance project...
  
  

Further reading

• Stan Malevanny. Case Study: Software Project Cost Estimates Using COCOMO II Model, 2005.

External links

• COCOMO II website
• NASA basic COCOMO online calculator
• COCOMO 81 Calculator