Lecture 8 Cocomo II

8/11/2019 Lecture 8 Cocomo II

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COCOMO IILecture 8

Asma Sajid


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COCOMO Introduction

The Constructive Cost Model(COCOMO) is an algorithmic softwarecost estimation model developed by Barry W. Boehm.

The model uses a basic regression formula with parameters that arederived from historical project data and current as well as futureproject characteristics.

COCOMO was first published in Boehm's (1981) as a model forestimating effort, cost, and schedule for software projects.

It drew on a study of 63 projects at TRW Aerospace where Boehmwas 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 rangingfrom assembly to PL/I.

These projects were based on the waterfall model of softwaredevelopment which was the prevalent software development processin 1981.


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Levels of Model

Boehm proposed three levels of the model: basic, intermediate,detailed. The basicCOCOMO'81 model is a

single-valued,

static model that computes software development effort (and cost)as a function of program size expressed in estimated thousanddelivered source instructions (KDSI).

The intermediateCOCOMO'81 model computes software development effort as a function of

program size and

a set of fifteen "cost drivers" that include subjective assessments ofproduct, hardware, personnel, and project attributes.

The advanced or detailed COCOMO'81 model incorporates all characteristics of the intermediate version

with an assessment of the cost drivers impact on each step(analysis, design, etc.) of the software engineering process.


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Modes of development

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-detachedprojects.(hardware, software, operational, ...)


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Modes of development


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Basic COCOMO

The basic COCOMO applies the parameterized equation without much

detailed consideration of project characteristics.

COCOMO'81 models depends on the two main equations:


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Drawbacks of COCOMO 81

It is hard to accurately estimate KDSI early on inthe project, when most effort estimates arerequired.

KDSI, actually, is not a size measure it is a lengthmeasure.

Extremely vulnerable to mis-classification of thedevelopment mode.

Success depends largely on tuning the model tothe needs of the organization, using historicaldata which is not always available


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What COCOMO 81 Needs?

New software processes

New phenomena: size, reuse

Need for decision making based on incompleteinformation.


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COCOMO 81 vs. COCOMO IIThe major differences between COCOMO I AND COCOMO II are:

1. COCOMO'81 requires software size in KDSI as an input, but COCOMO II is based on KSLOC(logical code). The major difference between DSI and SLOC is that a single Source Line of Code

may be several physical lines. For example, an "if-then-else" statement would be counted as

one SLOC, but might be counted as several DSI.

2. COCOMO II addresses the following three phases of the spiral life cycle: applications

development, early design and post architecture

3. COCOMO'81 provides point estimates of effort and schedule, but COCOMO II provides likely

ranges of estimates that represent one standard deviation around the most likely estimate.

4. The estimation equation exponent is determined by five scale factors (instead of the three

5. development modes)

6. Changes in cost drivers are:

1. Added cost drivers (7): DOCU, RUSE, PVOL, PLEX, LTEX, PCON, SITE

2. Deleted cost drivers (5): VIRT, TURN, VEXP, LEXP, MODP

3. Alter the retained ratings to reflect more up-do-date software practices

7. Data points in COCOMO I: 63 and COCOMO II: 161

8. COCOMO II adjusts for software reuse and reengineering where automated tools are used for

translation of existing software, but COCOMO'81 made little accommodation for these factors.

9. COCOMO II accounts for requirements volatility in its estimates

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Lecture 8 Cocomo II