The Constructive Cost Model (COCOMO) is an algorithmic software cost estimation model developed by Barry Boehm. The model uses a basic regression formula, with parameters that are derived from historical project data and current project characteristics.
COCOMO was first published in 1981 Barry W. Boehm's 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 Barry Boehm was Director of Software Research and Technology in 1981. 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 1997 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.
Basic COCOMO computes software development effort (and cost) as a function of program size. Program size is expressed in estimated thousands of lines of code (KLOC).
COCOMO applies to three classes of software projects:
The basic COCOMO equations take the form
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:-
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.
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 1. 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.