1. Software Engineering: A Practitioner’s Approach
Chapter 23
Estimation
Software Engineering: A Practitioner’s Approach
6th Edition
Roger S. Pressman

2. Software Project Estimation (1)
S/W is the most expensive element of virtually all computer based systems
S/W cost and effort estimation will never be an exact science
Too many variables
Human
Technical
Environmental
Political

3. Software Project Estimation (2)
Options for estimation
Delay estimation until late in the project
Attractive, but not practical
Base estimates on similar projects that have already been completed
Unfortunately, past experience has not always been a good indicator of future results
Use relatively simple decomposition techniques to generate project cost and effort estimates
“Divide and conquer” approach
Use one or more empirical models for software cost and effort estimation
Can be used as a cross-check for the previous option and vice versa

4. Decomposition Techniques
Two different points of view for the decomposition approach
Decomposition of the problem
Decomposition of the process
But first, the project planner must
Understand the scope of the s/w to be built
Generate an estimate of its “size”

5. Software Sizing
Sizing represents the project planner’s first major challenge
Size refers to a quantifiable outcome of the s/w project (e.g. LOC and/or FP)
Four different approaches to the sizing problem [PUT92]
“Fuzzy Logic” sizing
Function point sizing
Standard component sizing
Change sizing

6. An Example of LOC-Based Estimation (1)
Function
Estimated LOC
User interface and control facilities (UICF)
Two-dimensional geometric analysis (2DGA)
Three-dimensional geometric analysis (3DGA)
Database management (DBM)
Computer graphics display facilities (CGDF)
Peripheral control function (PCF)
Design analysis modules (DAM)
2,300
5,300
6,800
3,350
4,950
2,100
8,400
Estimated lines of code
33,200

7. An Example of LOC-Based Estimation (2)
Estimated lines of code = W = 33,200
Let,
Average productivity = 620 LOC/pm = X
Labor rate = $8,000 per month = Y
So,
Cost per line of code = Z = Y/X = $13 (approx.)
Total estimated project cost = W*Z = $431,000 (approx.)
Estimated effort = W/X = 54 person-months (approx)

8. An Example of FP-Based Estimation (1)
Information Domain Value
Count
Weighting factor
Simple
Average
Complex
External Inputs (EIS) 3 X 4 6 = 9
External Outputs (EOs) 2 5 7 8
External Inquiries (EQs)
Internal Logical Files (ILFs) 1 10 15
External Interface Files (EIFs) 20
Count Total 50
Figure 15.4: Computing function points

9. An Example of FP-Based Estimation (2)

10. An Example of FP-Based Estimation (3)
Value Adjustment Factors
1.17

11. An Example of FP-Based Estimation (4)
Now,
FPestimated = count-total  [   (Fi)]
Fi (i = 1 to 14 are value adjustment factors)
So,
FPestimated = W = 320  [  52] = 375 (approx.)
Let,
Average Productivity = X = 6.5 FP/pm
Labor rate = Y = $8,000 per month
Cost per FP = Z = Y/X = $1,230 (approx.)
Total estimated project cost = W*Z = $461,000 (approx.)
Estimated effort = W/X = 58 person-months (approx)

12. Empirical Estimation Models
Uses empirically derived formulas to predict effort as a function of LOC or FP
The empirical data are derived from a limited sample of projects
So, no estimation model is appropriate for all classes of s/w and in all development environments
The results obtained from such models must be used judiciously
An estimation model should be calibrated to reflect local conditions

13. The Structure of Estimation Models (1)
Derived using regression analysis on data collected from past s/w projects
Overall structure, E = A + B  (ev)c
Here,
A, B, and C are empirically derived constants
E is effort in person-months
ev is the estimation variable (either LOC or FP)
Some form of project adjustment component is also used

14. The Structure of Estimation Models (2)
Example of a LOC-oriented estimation model (Bailey-Basili model)
E =  (KLOC)1.16
Example of a FP-oriented estimation model (Kemerer model)
E = FP
Estimation models must be calibrated for local needs.

15. The COCOMO II Model (1) COnstructive COst Model
A hierarchy of estimation models
Addresses the following areas
Application composition model
Early design stage model
Post-architecture stage model
Three different sizing options are available
Object points
Function points
Lines of source code

16. The COCOMO II Model (2)
The COCOMO II application composition model uses object points
Object point is computed using counts of the number of
Screens (at the user interface)
Reports
Components likely to be required to build the application

17. Figure 23.6: Complexity weighting for object types
The COCOMO II Model (3)
Object type
Complexity weight
Simple
Medium
Difficult
Screen 1 2 3
Report 5 8
3GL component 10
Figure 23.6: Complexity weighting for object types

18. Figure 23.7: Productivity rate for object points
The COCOMO II Model (4)
Developer’s experience/capability
Very low
Low
Nominal
High
Very High
Environment maturity/capability
PROD 4 7 13 25 50
Figure 23.7: Productivity rate for object points

19. The COCOMO II Model (5) NOP = (object points)  [(100-%reuse)/100]
NOP = New Object Points
Object Points = Weighted Total
%reuse = Percent of reuse
Estimated effort = NOP/PROD
PROD = Productivity Rate
PROD = NOP/person-month

20. The Software Equation [PUT92] (1)
It is a multivariate model
Has been derived from productivity data collected for over 4000 contemporary s/w projects
E = [LOC  B0.333/P]3  (1/t4)
Here,
E = effort in person-months or person-years
t = project duration in months or years
B = “special skills factor”
P = “productivity parameter”

21. The Software Equation [PUT92] (2)
For small programs (KLOC = 5 to 15)
B = 0.16
For programs greater than 70 KLOC
B = 0.39
P = 2000 for development of real-time embedded s/w
P = 10,000 for telecommunication and systems s/w
P = 28,000 for business systems applications

22. The Software Equation [PUT92] (3)
Simplified formulas
tmin = 8.14 (LOC/P)0.43 in months for tmin > 6 months
tmin = minimum development time
E = 180 Bt3 in person-months for E  20 person-months
Here t is represented in years

23. The Make/Buy Decision S/W acquisition options
s/w may be purchased (or licensed) off the shelf
“full-experience” or “partial-experience” s/w components may be acquired and then modified and integrated to meet specific needs
s/w may be custom built by an outside contractor to meet the purchaser’s specifications

24. Creating a Decision Tree

25. Computing Expected Cost
(path probability) x (estimated path cost) i i
For example, the expected cost to build is:
expected cost = 0.30 ($380K) ($450K)
build
= $429 K
similarly,
expected cost = $382K
reuse
expected cost = $267K
buy
expected cost = $410K
contr

26. Outsourcing
S/W engineering activities are contracted to a third party who does the work at lower cost and, hopefully, higher quality
S/W work conducted within a company is reduced to a contract management activity
The decision to outsource can be either strategic or tactical
Has both merits and demerits

27. Chapter 23
23.5, 23.6, , ,
23.7
23.10
Exercises-
23.4, 23.5, 23.7