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Ch23Estimation

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Ch23EstimationnullChapter 23 – Project planningChapter 23 – Project planning-Estimation techniquesEstimation techniquesEstimation techniquesOrganizations need to make software effort and cost estimates. There are two types of technique that can be used to do this: Experienc...

Ch23Estimation
nullChapter 23 – Project planningChapter 23 – Project planning-Estimation techniquesEstimation techniquesEstimation techniquesOrganizations need to make software effort and cost estimates. There are two types of technique that can be used to do this: Experience-based techniques The estimate of future effort requirements is based on the manager’s experience of past projects and the application domain. Essentially, the manager makes an informed judgment of what the effort requirements are likely to be. Algorithmic cost modeling In this approach, a formulaic approach is used to compute the project effort based on estimates of product attributes, such as size, and process characteristics, such as experience of staff involved. Experience-based approachesExperience-based approachesExperience-based techniques rely on judgments based on experience of past projects and the effort expended in these projects on software development activities. Typically, you identify the deliverables to be produced in a project and the different software components or systems that are to be developed. You document these in a spreadsheet, estimate them individually and compute the total effort required. It usually helps to get a group of people involved in the effort estimation and to ask each member of the group to explain their estimate. Algorithmic cost modellingAlgorithmic cost modellingCost is estimated as a mathematical function of product, project and process attributes whose values are estimated by project managers: Effort = A ´ SizeB ´ M A is an organisation-dependent constant, B reflects the disproportionate effort for large projects and M is a multiplier reflecting product, process and people attributes. The most commonly used product attribute for cost estimation is code size. Most models are similar but they use different values for A, B and M.Estimation accuracyEstimation accuracyThe size of a software system can only be known accurately when it is finished. Several factors influence the final size Use of COTS and components; Programming language; Distribution of system. As the development process progresses then the size estimate becomes more accurate. The estimates of the factors contributing to B and M are subjective and vary according to the judgment of the estimator.Estimate uncertainty Estimate uncertainty The COCOMO 2 modelThe COCOMO 2 modelAn 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 2 modelsCOCOMO 2 modelsCOCOMO 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.COCOMO estimation models COCOMO estimation models Application composition modelApplication composition modelSupports 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. Formula is 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.Application-point productivityApplication-point productivityEarly design modelEarly design modelEstimates 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.MultipliersMultipliersMultipliers 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 modelThe reuse modelTakes 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 1Reuse model estimates 1For 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 2Reuse model estimates 2When 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 levelPost-architecture levelUses 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 termThis depends on 5 scale factors (see next slide). Their sum/100 is added to 1.01 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. The company has a CMM level 2 rating. Precedenteness - new project (4) Development flexibility - no client involvement - Very high (1) Architecture/risk resolution - No risk analysis - V. Low .(5) Team cohesion - new team - nominal (3) Process maturity - some control - nominal (3) Scale factor is therefore 1.17. The exponent termScale factors used in the exponent computation in the post-architecture model Scale factors used in the exponent computation in the post-architecture model MultipliersProduct attributes Concerned with required characteristics of the software product being developed. Computer attributes Constraints imposed on the software by the hardware platform. Personnel attributes Multipliers that take the experience and capabilities of the people working on the project into account. Project attributes Concerned with the particular characteristics of the software development project. MultipliersThe effect of cost drivers on effort estimates The effect of cost drivers on effort estimates The effect of cost drivers on effort estimates The effect of cost drivers on effort estimates Project duration and staffingProject duration and staffingAs 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.Key pointsKey pointsEstimation techniques for software may be experience-based, where managers judge the effort required, or algorithmic, where the effort required is computed from other estimated project parameters. The COCOMO II costing model is an algorithmic cost model that uses project, product, hardware and personnel attributes as well as product size and complexity attributes to derive a cost estimate.
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