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Software Engineering
(2160701)
SE2160701
Summer2018
Question2b
BE  Semester
Summer2018

04/28/2018
Goto Question Paper
Q2) (b)
Explain COCOMO model for project estimation
4 Marks
COCOMO (Constructive Cost Estimation Model) was proposed by Boehm
According to Boehm, software cost estimation should be done through three stages:
Basic COCOMO,
Intermediate COCOMO, and
Complete COCOMO
Basic COCOMO Model
The basic COCOMO model gives an approximate estimate of the project parameters
The basic COCOMO estimation model is given by the following expressions
Effort=a
_{1}
+(KLOC)
^{a2}
PM

Tdev=b
_{1}
*(Effort)
^{b2}
Months
KLOC is the estimated size of the software product expressed in Kilo Lines of Code,
a1, a2, b1, b2 are constants for each category of software products,
Tdev is the estimated time to develop the software, expressed in months,
Effort is the total effort required to develop the software product, expressed in person months (PMs).
Project
a
_{1}
a
_{2}
b
_{1}
b
_{2}
Organic
2.5
1.05
2.5
0.38
Semidetached
3.0
1.12
2.5
0.35
Embedded
3.6
1.20
2.5
0.32
The effort estimation is expressed in units of personmonths (PM)
It is the area under the personmonth plot (as shown in fig.)
An effort of 100 PM
does not imply that 100 persons should work for 1 month
does not imply that 1 person should be employed for 100 months
it denotes the area under the personmonth curve (fig.)
(Figure: Person Month Curve)
Every line of source text should be calculated as one LOC irrespective of the actual number of instructions on that line
If a single instruction spans several lines (say n lines), it is considered to be nLOC
The values of a1, a2, b1, b2 for different categories of products (i.e. organic, semidetached, and embedded) as given by Boehm
He derived the expressions by examining historical data collected from a large number of actual projects
Insight into the basic COCOMO model can be obtained by plotting the estimated characteristics for different software sizes
Fig. shows a plot of estimated effort versus product size
From fig. we can observe that the effort is somewhat superlinear in the size of the software product
The effort required to develop a product increases very rapidly with project size
(Figure: Efforts vs Product Size)
The development time versus the product size in KLOC is plotted in fig.
From fig., it can be observed that the development time is a sublinear function of the size of the product
i.e. when the size of the product increases by two times, the time to develop the product does not double but rises moderately
From fig., it can be observed that the development time is roughly the same for all the three categories of products
(Figure: Development Time vs Size)
Effort and the duration estimations obtained using the COCOMO model are called as nominal effort estimate and nominal duration estimate
The term nominal implies that
if anyone tries to complete the project in a time shorter than the estimated duration, then the cost will increase drastically
But, if anyone completes the project over a longer period of time than the estimated, then there is almost no decrease in the estimated cost value