Term paper On

COCOMO Model

Introduction
The original COCOMO stands for Constructive Cost Model. The word "constructive" implies that the complexity of the model can be understood because of the openness of the model, which permits exactly to know WHY the model gives the estimates it does. The model was first published by Dr. Barry Boehm in 1981, and reflected the software development practices of these days. The name of this model was COCOMO I or COCOMO ’81. Since this time many efforts were done in the improvement of the software development techniques. Some of the changes were moving away from mainframe overnight batch processing to real time applications, strenuousness in effort in building software for reusing, new kind of system development in including off-the-shelf software components (COTS) and spending as much effort on designing and managing the software development process as was once spent creating the software product. This advanced version at present time is known as COCOMO II. The work for COCOMO II has been supported financially and technically by the COCOMO II Program Affiliates: Aerospace, Air Force Cost Analysis Agency, Allied Signal, DARPA, DISA, Draper Lab, EDS, E-Systems, FAA, Fidelity, GDE Systems, Hughes, IDA, IBM, JPL, Litton, Lockheed Martin, Loral, Lucent, MCC, MDAC, Microsoft, Motorola, Northrop Grumman, ONR, Rational, Raytheon, Rockwell, SAIC, SEI, SPC, Sun, TASC, Teledyne, TI, TRW, USAF Rome Lab, US Army Research Labs, US Army TACOM, Telcordia, and Xerox.

COCOMO I
Boehm proposed three levels of the model: basic, intermediate, detailed.  The basic COCOMO'81 model is a single-valued, static model that computes software development effort (and cost) as a function of program size expressed in estimated thousand delivered source instructions (KDSI).  The intermediate COCOMO'81 model computes software development effort as a function of program size and a set of fifteen "cost drivers" that include subjective assessments of product, 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 driver’s impact on each step (analysis, design, etc.) of the software engineering process. COCOMO I models depends on the two main equations. They are given below. 1. development effort : MM = a * KDSI b based on MM - man-month / person month / staffmonth is one month of effort by one person. In COCOMO I, there are 152 hours per Person month. According to organization this values may differ from the standard by 10% to 20%. 2. effort and development time (TDEV) : TDEV = 2.5 * MM c

The coefficents a, b and c depend on the mode of the development. There are three modes of development:

The basic COCOMO applies the parameterized equation without much detailed consideration of project characteristics.

The same basic equation for the intermediate model is used, but fifteen cost drivers are rated on a scale of 'very low' to 'very high' to calculate the specific effort multiplier and each of them returns an adjustment factor which multiplied yields in the total EAF (Effort Adjustment Factor). The adjustment factor is 1 for a cost driver that's judged as normal. In addition to the EAF, the model parameter "a" is slightly different in Intermediate COCOMO from the basic model. The parameter "b" remains the same in both models.

The Advanced COCOMO model computes effort as a function of program size and a set of cost drivers weighted according to each phase of the software lifecycle. The Advanced model applies

the Intermediate model at the component level, and then a phase-based approach is used to consolidate the estimate. The four phases used in the detailed COCOMO model are: requirements planning and product design (RPD), detailed design (DD), code and unit test (CUT), and integration and test (IT). Each cost driver is broken down by phases.

Advantages of COCOMO I  COCOMO is transparent, one can see how it works unlike other models such as SLIM.  Drivers are particularly helpful to the estimator to understand the impact of different factors that affect project costs. Drawbacks of COCOMO I  It is hard to accurately estimate KDSI early on in the project, when most effort estimates are required.  KDSI, actually, is not a size measure it is a length measure.  Extremely vulnerable to mis-classification of the development mode.  Success depends largely on tuning the model to the needs of the organization, using historical data which is not always available. 

COCOMO II MODEL
COCOMO II provides three stage series of models for estimation of software projects: Application Composition Model for earliest phases or spiral cycles [prototyping, and any other prototyping occurring later in the life cycle]. Early Design Model for next phases or spiral cycles. It involves exploration of architectural alternatives or incremental development strategies. Level of detail consistent with level of information available and the general level of estimation accuracy needed at this stage.

Post Architecture Model: once the project is ready to develop and sustain a fielded system it should have a life-cycle architecture, which provides more accurate information on cost driver inputs, and enables more accurate cost estimates. The major differences between COCOMO I AND COCOMO II  COCOMO I 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.  COCOMO II addresses the following three phases of the spiral life cycle: applications development, early design and post architecture.  COCOMO I 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.  The estimation equation exponent is determined by five scale factors (instead of the three development modes).  Changes in cost drivers are:  Added cost drivers (7): DOCU, RUSE, PVOL, PLEX, LTEX, PCON, SITE  Deleted cost drivers (5): VIRT, TURN, VEXP, LEXP, MODP  Alter the retained ratings to reflect more up-do-date software practices  Data points in COCOMO I: 63 and COCOMO II: 161.  COCOMO II adjusts for software reuse and reengineering where automated tools are used for translation of existing software, but COCOMO I made little accommodation for these factors COCOMO II accounts for requirements volatility in its estimates. Methodology Preparation includes: Parameter definition, how to use parameters to produce effective estimates. The methodology tries to minimize risk of lost expert time and maximize estimates. Here the question arises which parameters are significant (most estimate efficient), in which way and with which rating scale. Seven modeling steps are as follows. 1. analyse existing literature 2. review software cost modelling literature 3. to insight on improved functional forms potentially significant parameters 4. parameter definition issues (e.g. size) 5. identification of potential new parameters Process Maturity and Multisite Development 6. continuation of a number of parameters from COCOMO I 7. dropping of such COCOMO I parameters as turnaround time and modern programming practices (subsumed by process maturity).

COCOMO II Effort Equation The COCOMO II model makes its estimates of required effort (measured in Person-Months PM) based primarily on your estimate of the software project's size (as measured in thousands of SLOC, KSLOC)): Effort = 2.94 * EAF * (KSLOC)E where, EAF is the Effort Adjustment Factor derived from the Cost Drivers and E is an exponent derived from the five Scale Drivers (Precedentedness, Development Flexibility, Architecture / Risk Resolution, Team Cohesion, Process Maturity). As an example, a project with all Nominal Cost Drivers and Scale Drivers would have an EAF of 1.00 and exponent, E, of 1.0997. Assuming that the project is projected to consist of 8,000 source lines of code, COCOMO II estimates that 28.9 Person-Months of effort is required to complete it: Effort = 2.94 * (1.0) * (8)1.0997 = 28.9 Person-Months

Effort Adjustment Factor The Effort Adjustment Factor in the effort equation is simply the product of the effort multipliers corresponding to each of the cost drivers for your project. For example, if your project is rated Very High for Complexity (effort multiplier of 1.34), and Low for Language & Tools Experience (effort multiplier of 1.09), and all of the other cost drivers are rated to be Nominal (effort multiplier of 1.00), the EAF is the product of 1.34 and 1.09. Effort Adjustment Factor = EAF = 1.34 * 1.09 = 1.46 Effort = 2.94 * (1.46) * (8)1.0997 = 42.3 Person-Months

COCOMO II Schedule Equation The COCOMO II schedule equation predicts the number of months required to complete your software project. The duration of a project is based on the effort predicted by the effort equation:

Duration = 3.67 * (Effort) SE

where, Effort is the effort from the COCOMO II effort equation and SE is the schedule equation exponent derived from the five Scale Drivers (Precedentedness, Development Flexibility, Architecture / Risk Resolution, Team Cohesion, Process Maturity). Continuing the above example, and substituting the exponent of 0.3179 that is calculated from the scale drivers, yields an estimate of just over a year, and an average staffing of between 3 and 4 people: Duration = 3.67 * (42.3)0.3179 = 12.1 months Average staffing = (42.3 Person-Months) / (12.1 Months) = 3.5 people

Advantages of COCOMO II  COCOMO II is an industry standard  very profound information is easy available  clear and effective calibration process by combining delphi with algorithmic cost estimation techniques (Bayesian method)  backwards compatibility' with the Rosetta Stone  various extension for almost every purpose are available  Tool support (also for the various extensions)

Disadvantages of COCOMO II  The 'heart' of COCOMO II is still based on a waterfall process model (predilection)  most of the extensions are still experimental and not fully calibrated till now.  duration calculation for small projects is unreasonable.

Conclusion
Software cost estimation is an important part of the software development process. The COCOMO suite (COCOMO II model and its extensions) offers a powerful instrument to predict software costs. Unfortunately not all of the extensions are already calibrated and therefore still experimental. Only the Post-Architecure model is implemented in a calibrated software tool. Despite this disadvantage the COCOMO II suite helps managing software projects. It supports process improvement analyses, tool purchases, architecture changes, component make/buy tradeoffs and decision making process with credible results. Many endeavours were done to

measure up to the changes in software life cycles, technologies, components, tools, notations and organizational cultures since the first version of COCOMO (COCOMOI, COCOMO 81).

References
[1] http//sunset.usc.edu/COCOMO II/expert_COCOMO/expert_COCOMO.html. [2] http://sunset.usc.edu/research/cocomosuite/index.html [3] www.softstarsystems.com/overview.html [4] www.csi.uottawa.ca/~bob/seg33002004/CocomoModels.ppt [5] www.davidpublishing.com/.../Upfile/12/.../2012121105997080.pdf [6] www.mhhe.com/engcs/compsci/pressman/.../olc/COCOMO.html [7] https://lirias.kuleuven.be/bitstream/123456789/.../1/KBI_0829.pdf