Slide 1: 1 SOFTWARE PROJECT MANAGEMENT Course Notes Chapter 5
Software effort estimation The COCOMO model : The COCOMO model An empirical model based on project experience.
Well-documented, ‘independent’ model which is not tied to a specific software vendor.
Long history from initial version published in 1981 (COCOMO-81) through various instantiations to COCOMO 2.
COCOMO 2 takes into account different approaches to software development, reuse, etc. COCOMO 81 : COCOMO 81 COCOMO 2 : COCOMO 2 COCOMO 81 was developed with the assumption that a waterfall process would be used and that all software would be developed from scratch.
Since its formulation, there have been many changes in software engineering practice and COCOMO 2 is designed to accommodate different approaches to software development. COCOMO 2 models : COCOMO 2 models COCOMO 2 incorporates a range of sub-models that produce increasingly detailed software estimates.
The sub-models in COCOMO 2 are:
Application composition model. Used when software is composed from existing parts.
Early design model. Used when requirements are available but design has not yet started.
Reuse model. Used to compute the effort of integrating reusable components.
Post-architecture model. Used once the system architecture has been designed and more information about the system is available. Use of COCOMO 2 models : Use of COCOMO 2 models Application composition model : Application composition model Supports prototyping projects and projects where there is extensive reuse.
Based on standard estimates of developer productivity in application (object) points/month.
Takes CASE tool use into account.
PM = ( NAP ´ (1 - %reuse/100 ) ) / PROD
PM is the effort in person-months, NAP is the number of application points and PROD is the productivity. Object point productivity : Object point productivity Early design model : Early design model Estimates can be made after the requirements have been agreed.
Based on a standard formula for algorithmic models
PM = A ´ SizeB ´ M where
M = PERS ´ RCPX ´ RUSE ´ PDIF ´ PREX ´ FCIL ´ SCED;
A = 2.94 in initial calibration, Size in KLOC, B varies from 1.1 to 1.24 depending on novelty of the project, development flexibility, risk management approaches and the process maturity. Multipliers : Multipliers Multipliers reflect the capability of the developers, the non-functional requirements, the familiarity with the development platform, etc.
RCPX - product reliability and complexity;
RUSE - the reuse required;
PDIF - platform difficulty;
PREX - personnel experience;
PERS - personnel capability;
SCED - required schedule;
FCIL - the team support facilities. The reuse model : The reuse model Takes into account black-box code that is reused without change and code that has to be adapted to integrate it with new code.
There are two versions:
Black-box reuse where code is not modified. An effort estimate (PM) is computed.
White-box reuse where code is modified. A size estimate equivalent to the number of lines of new source code is computed. This then adjusts the size estimate for new code. Reuse model estimates 1 : Reuse model estimates 1 For generated code:
PM = (ASLOC * AT/100)/ATPROD
ASLOC is the number of lines of generated code
AT is the percentage of code automatically generated.
ATPROD is the productivity of engineers in integrating this code. Reuse model estimates 2 : Reuse model estimates 2 When code has to be understood and integrated:
ESLOC = ASLOC * (1-AT/100) * AAM.
ASLOC and AT as before.
AAM is the adaptation adjustment multiplier computed from the costs of changing the reused code, the costs of understanding how to integrate the code and the costs of reuse decision making. Post-architecture level : Post-architecture level Uses the same formula as the early design model but with 17 rather than 7 associated multipliers.
The code size is estimated as:
Number of lines of new code to be developed;
Estimate of equivalent number of lines of new code computed using the reuse model;
An estimate of the number of lines of code that have to be modified according to requirements changes. The exponent term : This depends on 5 scale factors
A company takes on a project in a new domain. The client has not defined the process to be used and has not allowed time for risk analysis.
Precedenteness - new project
Development flexibility - no client involvement - Very high
Architecture/risk resolution - No risk analysis - V. Low .
Team cohesion - new team - nominal
Process maturity - some control - nominal The exponent term Exponent scale factors : Exponent scale factors Multipliers : Product attributes
Concerned with required characteristics of the software product being developed.
Constraints imposed on the software by the hardware platform.
Multipliers that take the experience and capabilities of the people working on the project into account.
Concerned with the particular characteristics of the software development project. Multipliers Effects of cost drivers : Effects of cost drivers Project planning : Algorithmic cost models provide a basis for project planning as they allow alternative strategies to be compared.
Embedded spacecraft system
Must be reliable;
Must minimise weight (number of chips);
Multipliers on reliability and computer constraints
Development effort. Project planning Management options : Management options Management option costs : Management option costs Option choice : Option choice Option D (use more experienced staff) appears to be the best alternative
However, it has a high associated risk as experienced staff may be difficult to find.
Option C (upgrade memory) has a lower cost saving but very low risk.
Overall, the model reveals the importance of staff experience in software development. Project duration and staffing : Project duration and staffing As well as effort estimation, managers must estimate the calendar time required to complete a project and when staff will be required.
Calendar time can be estimated using a COCOMO 2 formula
TDEV = 3 ´ (PM)(0.33+0.2*(B-1.01))
PM is the effort computation and B is the exponent computed as discussed above (B is 1 for the early prototyping model). This computation predicts the nominal schedule for the project.
The time required is independent of the number of people working on the project. Staffing requirements : Staffing requirements Staff required can’t be computed by diving the development time by the required schedule.
The number of people working on a project varies depending on the phase of the project.
The more people who work on the project, the more total effort is usually required.
A very rapid build-up of people often correlates with schedule slippage. Key points : Key points There is not a simple relationship between the price charged for a system and its development costs.
Factors affecting productivity include individual aptitude, domain experience, the development project, the project size, tool support and the working environment.
Software may be priced to gain a contract and the functionality adjusted to the price. Key points : Key points Different techniques of cost estimation should be used when estimating costs.
The COCOMO model takes project, product, personnel and hardware attributes into account when predicting effort required.
Algorithmic cost models support quantitative option analysis as they allow the costs of different options to be compared.
The time to complete a project is not proportional to the number of people working on the project.