COCOMO for Cost Estimation

8/10/2019 COCOMO for Cost Estimation

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COCOMO for Cost Estimation


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The Standish Group Example

The Standish Groups reports on projectmanagement chaos contain some of the mostrespected statistics about software project failures.According to the Standish group, the average projectexceeds its budget by 90%, and its schedule by 120%.Furthermore, over 30% of projects are cancelledbefore completion. In the United States alone, thisrepresents about $100 billion in wasted money per

year. (http://www.standishgroup.com)
http://www.standishgroup.com/http://www.standishgroup.com/


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General

Accurately forecasting how much time and

effort it will take to develop a system, or to

take a specific set of changes is one of the

biggest challenges in SE.

Although a lot of projects underestimated the

cost they needed, cost estimation is still

necessary for a new project.


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Types of Cost

Time: The difference in time from the start

date to the end date of a project. This is

called elapsed time.

Effort: Also called development effort. This is

the amount of labor needed for a project.

Effort is expressed inperson-monthsmostly.


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From Effort to Money

1. Find the average salaryin the company.

2. Calculate the weighted average costbymultiplying the average salary by a weight,

determined from the expenses onemployees (the number is usually between 2and 3).

3. Cost of the project = effort * weightedaverage cost


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COCOMO

Constructive Cost Model (COCOMO) proposed

by Barry Boehm.

It is a cost estimation model that considers the

complexity of a project:

Organic

Semidetached

embedded


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Organic

In the organic mode, relatively small software teamsdevelop software in a highly familiar, in-houseenvironment. Most people connected with theproject have extensive experience in working withrelated systems within the organization, and have athorough understanding of how the system underdevelopment will contribute to the organizationsobjectives. Very few organic-mode projects have

developed products with more than 50 thousanddelivered source instructions (KDSI).


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Semidetached

The semidetached mode of software developmentrepresents an intermediate stage between theorganic and embedded modes. "Intermediate"may mean either of two things:

1.An intermediate level of project characteristic.

2.A mixture of the organic and embedded modecharacteristics.

The size range of a semidetached modeproduct generally extends up to 300 KDSI.


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Embedded

The major distinguishing factor of anembedded-mode software project is a need tooperate within tight constraints. The product

must operate within (is embedded in) astrongly coupled complex of hardware,software, regulations, and operationalprocedures, such as an electronic funds

transfer system or an air traffic control system.


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Basic COCOMO

Basic COCOMO model estimates the software

development effort using only a single

predicator variable (size in DSI) and three

software development modes.

Development Mode Effort Equation Schedule Equation

Organic E=2.4*(KDSI)^1.05 TDEV=2.5*(E)^0.38

Semidetached E=3.0*(KDSI)^1.12 TDEV=2.5*(E)^0.35

Embedded E=3.6*(KDSI)^1.20 TDEV=2.5*(E)^0.32


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Basic COCOMO

Basic COCOMO is good for quick, early, rough

order of magnitude estimates of software

costs, but its accuracy is necessarily limited

because of its lack of factors which have asignificant influence on software costs.


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An Example

Project A is to be a 32,000 DSI semi-detached

software. Then we can estimate:

Effort = 3.0*(32)^1.12 = 146 person months

Productivity = 32,000 DSI/ 146 PM = 219 DSI/PM

Schedule = 2.5*(146)^0.35 = 14 months

Average staffing = 146 pm/ 14 months = 10

Persons


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Intermediate COCOMO

The Intermediate COCOMO estimates the softwaredevelopment effort by using 15 cost driver variablesbesides the side variable used in Basic COCOMO.

The Intermediate COCOMO can be applied across theentire software product for easily and roughly costestimation during the early stage, or be applied atthe software product component level for moreaccurate cost estimation in more detailed stages.


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Intermediate COCOMO

The Intermediate COCOMO uses an Effort

Adjustment Factor (EAF) and slightly different

coefficients for the effort equations than the

Basic COCOMO.

The EAF is simply the product of the Effort

Multipliers corresponding to each of the cost

drives for the target product.


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Intermediate COCOMO

Development Mode Effort Equation Schedule Equation

Organic E=EAF*3.2*(KDSI)^1.05 TDEV=2.5*(E)^0.38

Semidetached E=EAF*3.0*(KDSI)^1.12 TDEV=2.5*(E)^0.35

Embedded E=EAF*2.8*(KDSI)^1.20 TDEV=2.5*(E)^0.32


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EAF Features

Product attributes

Required software reliability(RELY)The extent to

which the software product must perform its intended

functions satisfactorily over a period of time.

Database size(DATA)The degree of the total amount

of data to be assembled for the database.

Product complexity(CPLX)The level of complexity of

the product to be develop.


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EAF Features

Computer attributes

Execution time constraint(TIME)The degree of the execution

constraint imposed upon a software product.

Main storage constraint(STOR)The degree of main storage

constraint imposed upon a software product. Virtual machine volatility(VIRT)The level of the virtual machine

underlying the product to be developed.

Computer turnaround time(TURN)The level of computer

response time experienced by the project team developing the

product.


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EAF Features

Personal attributes

Analyst capability(ACAP)The level of capability of the analystsworking on a software product.

Applications experience(AEXP)The level of applicationsexperience of the project team developing the software product.

Programmer capability(PCAP)The level of capability of theprogrammers working on the software product.

Virtual machine experience(VEXP)The level of virtual machineexperience of the project team developing the product.

Programming language experience(LEXP)The level of

programming language experience of the project team developingthe product.


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EAF Features

Project attributes

Use of modern programming practices(MODP)The

degree to which modern programming practices are

used in developing software product.

Use of software tools(TOOL)The degree to which

software tools are used in developing the software

product.

Required development schedule(SCED)The level of

schedule constraint imposed upon the project team

developing the software product.


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EAF Rating

Cost Dr ivers Very Low Low Nominal H igh Very H igh Extr a H igh

RELY 0.75 0.88 1.00 1.15 1.40

DATA 0.94 1.00 1.08 1.16

CPLX 0.70 0.85 1.00 1.15 1.30 1.65

TIME 1.00 1.11 1.30 1.66

STOR 1.00 1.06 1.21 1.56

VIRT 0.87 1.00 1.15 1.30

TURN 0.87 1.00 1.07 1.15

ACAP 1.46 1.19 1.00 0.86 0.71

AEXP 1.29 1.13 1.00 0.91 0.82

PCAP 1.42 1.17 1.00 0.86 0.70

VEXP 1.21 1.10 1.00 0.90

LEXP 1.14 1.07 1.00 0.95

MODP 1.24 1.10 1.00 0.91 0.82

TOOL 1.24 1.10 1.00 0.91 0.83

SCED 1.23 1.08 1.00 1.04 1.10


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An Example

Project B is in embedded mode and estimated to be10,000 delivered source instructions in length. ItsEAF is as:

Cost Drivers

RELY 1.15

DATA 0.94

CPLX 1.30

TIME 1.11

STOR 1.06

VIRT 1.00

TURN 1.00

ACAP 0.86

AEXP 1.00

PCAP 0.86 VEXP 1.10

LEXP 1.00

MODP 0.91

TOOL 1.10

SCED 1.00


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An Example

Nominal Effort = 2.8*(10)^1.20 = 44 person-

months

The product of the multipliers = 1.15**1.00

= 1.35

The estimated effort = 1.35*44 = 59 person-

months


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Detailed COCOMO

The Detailed COCOMO differs from the Intermediate

COCOMO in two aspects:

Phase sensitive effort multipliers

The effort multipliers for every cost drivers are different during thesoftware development phases. These multipliers are used to

determine the amount of effort required to complete each phase.

The Module-Subsystem-System Hierarchy

The Software product is estimated in the three level hierarchical

decomposition. The 15 EAF are related to module or subsystemlevel.


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Detailed COCOMO

Module Level Cost Drivers

CPLX, PCAP, VEXP, LEXP

Subsystem Level Cost Drivers

RELY, DATA, TIME, STOR, VIRT

The subsystem level cost drivers vary from

subsystem to subsystem, but tend to be the same

for all the modules within a subsystem.


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Detailed COCOMO

The Detailed COCOMO uses the same equations forestimation as the Intermediate COCOMO.

Detailed Model uses a very complex procedure to estimatethe cost of the product. The procedure uses the DSIs for

subsystems and modules, and module level and subsystemlevel effort multipliers as inputs.

The Detailed COCOMO can estimate the staffing, cost, andduration of each of the development phases, subsystems,modulesallowing you to experiment with different

development strategies, to find the plan that best suits yourneeds and resources.


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Summary for COCOMO

All the cost models are adjunct to, not substitute for,

a detailed estimate by task by the project

management.

All the cost estimation models highly depends on theusers knowledge of application domain, analysis

ability and understanding of the cost model itself.

The process of meeting the estimate is equally

critical to the process of making the estimate.

COCOMO for Cost Estimation

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