SE 616 – Introduction to Software Engineering
· COCOMO is a model designed by Barry Boehm to give an estimate of the number of programmer-months it will take to develop a software product.
· This "COnstructive COst MOdel" is based on a study of about sixty projects at TRW, a Californian automotive and IT company, acquired by Northrop Grumman in late 2002. The programmes examined ranged in size from 2000 to 100,000 lines of code, and programming languages used ranged from assembly to PL/I.
· Cocomo consists of a hierarchy of three increasingly detailed and accurate forms.
Important observation: personnel motivation overwhelms all other parameters.
· suggests that leadership and teamsmanship are the most important skills of all, but this point was largely ignored.
Text-based user interface
expected cost = SUM[ (path probability)i x (estimated path cost)i ]
For example, the expected cost to build is:
expected costbuild = 0.30($380K)+0.70($450K) = $429 K
expected costreuse = $382K
expected costbuy = $267K
expected costcontract = $410K
Causes of Software Lateness:
· Software project scheduling - activity that distributes estimated effort across the planned project duration by allocating the effort to specific software engineering tasks.
· Schedule evolves over time.
o Early stages of planning - macroscopic schedule - identifies all major software engineering activities and the product functions to which they are applied.
o Project under way - detailed schedule - each entry on the macroscopic schedule is refined. Here, s
§ specific software tasks are identified and scheduled.
· Two Points of view for Scheduling
· Principles guiding software project scheduling:
o Compartmentalization. The project must be compartmentalized into a number of manageable activities and tasks.
o Interdependency. The interdependency of each compartmentalized activity or task must be determined. Some tasks must occur in sequence while others can occur in parallel.
o Time allocation.
§ Each task to be scheduled allocated some number of work units
§ Each task assigned a start date and a completion date that are a function of the interdependencies and whether work will be conducted on a full-time or part-time basis.
o Effort validation.
§ Every project has a defined number of staff members.
§ As time allocation occurs, the project manager ensures that no more than the allocated number of people has been scheduled at any given time.
o Defined responsibilities. Every task that is scheduled should be assigned to a specific team member.
o Defined outcomes. Every task that is scheduled should have a defined outcome. For software projects, the outcome is normally a work product or a part of a work product.
o Defined milestones. Every task or group of tasks should be associated with a project milestone. A milestone is accomplished when one or more work products has been reviewed for quality and has been approved.
· 40–20–40 rule - Recommended distribution of effort across the definition and development phases.
o Forty percent of all effort is allocated to front-end analysis and design.
o Forty percent is applied to back-end testing.
o Twenty percent is applied to coding.
· Task set - collection of software engineering work tasks, milestones, and deliverables that must be accomplished to complete a particular project.
· Task sets are designed to accommodate different types of projects and different degrees of rigor.
o determine type of project
o assess the degree of rigor required
o identify adaptation criteria
select appropriate software engineering tasks
· Most software organizations encounter the following projects:
· Degree of rigor is a function of many project characteristics.
Four different degrees of rigor can be defined:
· Project manager must develop a systematic approach for selecting the degree of rigor that is appropriate for a particular project.
· To accomplish this, project adaptation criteria are defined and a task set selector value is computed
· Adaptation criteria are used to determine the recommended degree of rigor with which the software process should be applied on a project.
Eleven adaptation criteria are defined for software projects:
· Each adaptation criterion is assigned a grade that ranges between 1 and 5
· To select the appropriate task set for a project, the following steps should be conducted:
o Enter grade into table.
o Value of a weighting factor ranges from 0.8 to 1.2 and provides an indication of the relative importance of a particular adaptation criterion to the types of software developed within the local environment.
o Entry point multiplier takes on a value of 0 or 1 and indicates the relevance of the adaptation criterion to the project type.
o Result of the product
grade x weighting factor x entry point multiplier
o Placed in the Product column of table for each adaptation criteria individually.
· This value will be used to help select the task set that is most appropriate for the project.
· Interpreting the TSS Value and Selecting the Task Set
· Once the task set selector is computed, the following guidelines can be used to select the appropriate task set for a project:
Task set selector value
Degree of rigor
TSS < 1.2
1.0 < TSS < 3.0
TSS > 2.4
· Overlap in TSS values from one recommended task set to another is intended to illustrate that sharp boundaries are impossible to define when making task set selections.
· The task set selector value, past experience, and common sense must all be factored into the choice of the task set for a project.
· Table illustrates how TSS might be computed for a hypothetical project.
o Project manager selects the grades shown in the Grade column.
o Project type is new application development.
o Therefore, entry point multipliers are selected from the NDev column. T
o he entry in the Product column is computed using
Grade x Weight x NewDev entry point multiplier
· Value of TSS (computed as the average of all entries in the product column) is 2.8.
· The manager has the option of using either the structured or the strict task set.
· Development projects are approached by applying the following major tasks:
1. Concept scoping determines the overall scope of the project.
2. Preliminary concept planning establishes the organization's ability to undertake the work implied by the project scope.
3. Technology risk assessment evaluates the risk associated with the technology to be implemented as part of project scope.
4. Proof of concept demonstrates the viability of a new technology in the software context.
5. Concept implementation implements the concept representation in a manner that can be reviewed by a customer and is used for "marketing" purposes when a concept must be sold to other customers or management.
6. Customer reaction to the concept solicits feedback on a new technology concept and targets specific customer applications.
· Development framework activities are iterative in nature.
· Actual development project might approach these activities in a number of planned increments, each designed to produce a deliverable that can be evaluated by the customer.
· Model Selection:
o Linear process model
o Evolutionary model
· Major tasks may be used to define a macroscopic schedule for a project.
· Macroscopic schedule must be refined to create a detailed project schedule.
· Refinement begins by taking each major task and decomposing it into a set of subtasks (with related work products and milestones).
Example of task decomposition - concept scoping for a development project:
· Individual tasks and subtasks have interdependencies based on their sequence.
· It is likely that development activities and tasks will be performed in parallel.
· Concurrent tasks must be coordinated so that they will be complete when later tasks require their work product(s).
· Task network ( activity network ) is a graphic representation of the task flow for a project.
o The task network depicts major software engineering tasks.
· Parallel tasks occur asynchronously
o Planner must determine intertask dependencies to ensure continuous progress toward completion.
o Project manager should be aware of those tasks that must be completed on schedule if the project as a whole is to be completed on schedule.
· Generalized project scheduling tools and techniques can be applied with little modification to software projects.
· Program evaluation and review technique (PERT) and critical path method (CPM) are two project scheduling methods that can be applied to software development.
· Both techniques are driven by information already developed in earlier project planning activities:
o Estimates of effort
o A decomposition of the product function
o The selection of the appropriate process model and task set
o Decomposition of tasks
· Interdependencies among tasks may be defined using a task network.
· Tasks (work breakdown structure (WBS)) are defined for the product as a whole or for individual functions.
· PERT and CPM provide quantitative tools that allow the software planner to
· Important boundary times discerned from a PERT or CPM network:
1. the earliest time that a task can begin when all preceding tasks are completed in the shortest possible time
2. the latest time for task initiation before the minimum project completion time is delayed
3. the earliest finish—the sum of the earliest start and the task duration
4. the latest finish—the latest start time added to task duration
5. the total float—the amount of surplus time or leeway allowed in scheduling tasks so that the network critical path is maintained on schedule.
· Boundary time calculations lead to a determination of critical path and provide the manager with a quantitative method for evaluating progress as tasks are completed.
· PERT and CPM implemented in a variety of automated tools that are available for the personal computer
· Such tools are easy to use and make the scheduling methods described previously available to every software project manager
· Software project schedule - planner begins with a set of tasks
· Effort, duration, and start date are input for each task.
· Tasks may be assigned to specific individuals.
· Timeline chart, ( Gantt chart) generated.:
· The project schedule provides a road map for a software project manager.
· Project schedule defines the tasks and milestones that must be tracked and controlled as the project proceeds.
· Tracking can be accomplished in a number of different ways:
o Conducting periodic project status meetings in which each team member reports progress and problems.
o Evaluating the results of all reviews conducted throughout the software engineering process.
o Determining whether formal project milestones have been accomplished by the scheduled date.
o Comparing actual start-date to planned start-date for each project task listed in the resource table
o Meeting informally with practitioners to obtain their subjective assessment of progress to date and problems on the horizon.
· Time-boxing strategy recognizes that the complete product may not be deliverable by the predefined deadline.
o An incremental software paradigm is chosen and a schedule is derived for each incremental delivery.
o Tasks associated with each increment are time-boxed.
o The schedule for each task is adjusted by working backward from the delivery date for the increment.
o A "box" is put around each task.
o When a task hits the boundary of its time box (plus or minus 10 percent), work stops and the next task begins.
· Earned value analysis (EVA) - qualitative approaches to project tracking
o Provides a common value scale for every software project task, regardless of the type of work being performed.
o The total hours to do the whole project are estimated, and every task is given an earned value based on its estimated percentage of the total.
· Earned value is a measure of progress
o Able to assess the "percent of completeness" of a project using quantitative analysis
Following steps performed:
1. The budgeted cost of work scheduled (BCWS) is determined for each work task represented in the schedule.
1. The work (in person-hours or person-days) of each software engineering task is planned.
2. BCWSi is the effort planned for work task i.
3. To determine progress at a given point along the project schedule, the value of BCWS is the sum of the BCWSi values for all work tasks that should have been completed by that point in time on the project schedule.
2. The BCWS values for all work tasks are summed to derive the budget at completion, BAC. Hence,
BAC = S (BCWSk) for all tasks k
3. Next, the value for budgeted cost of work performed (BCWP) is computed.
1. The value for BCWP is the sum of the BCWS values for all work tasks that have actually been completed by a point in time on the project schedule.
· The distinction between BCWS and BCWP is that the former represents the budget of the activities that were planned to be completed and the latter represents the budget of the activities that actually were completed.
· Given values for BCWS, BAC, and BCWP, progress indicators can be computed:
Schedule performance index, SPI = BCWP/BCWS
Schedule variance, SV = BCWP - BCWS
· SPI is an indication of the efficiency with which the project is utilizing scheduled resources.
o An SPI value close to 1.0 indicates efficient execution of the project schedule.
· SV is an absolute indication of variance from the planned schedule.
· Percent scheduled for completion = BCWS/BAC
o provides an indication of the percentage of work that should have been completed by time t.
· Percent complete = BCWP/BAC
o provides a quantitative indication of the percent of completeness of the project at a given point in time, t.
· Actual cost of work performed, ACWP - the sum of the effort expended on work tasks that have been completed by a point in time on the project schedule.
o It is then possible to compute
Cost performance index, CPI = BCWP/ACWP
Cost variance, CV = BCWP - ACWP
o CPI close to 1.0 provides a strong indication that the project is within its defined budget.
o CV is an absolute indication of cost savings (against planned costs) or shortfall at a particular stage of a project.
· Throughout the software process, a project team errors associated with each work product.
· Error-related measures and resultant metrics are collected over many software projects, can be used as a baseline for comparison against error data collected in real time.
· Error tracking can be used as one means for assessing the status of a current project.
· Software team performs formal technical reviews (and, later, testing) to find and correct errors, E, in work products produced during software engineering tasks.
· Any errors that are not uncovered (but found in later tasks) are defects, D.
· Defect removal efficiency is defined as
DRE = E/(E + D)
· DRE is a process metric that provides a strong indication of the effectiveness of quality assurance activities
· DRE can also be used to assist a project manager in determining the progress that is being made as a software project moves through its scheduled work tasks.
o e.g. a software organization has collected error and defect data over the past 24 months and has developed averages for the following metrics:
§ Errors per requirements specification page, Ereq
§ Errors per component—design level, Edesign
§ Errors per component—code level, Ecode
§ DRE—requirements analysis
§ DRE—architectural design
§ DRE—component level design
· As the project progresses through each software engineering step, the software team records and reports the number of errors found during requirements, design, and code reviews.
· The project manager calculates current values for Ereq, Edesign, and Ecode.
· Compared to averages for past projects.
· If current results vary by more than 20% from the average, there may be cause for concern and there is certainly cause for investigation.
· For example, if Ereq = 2.1 for project X, yet the organizational average is 3.6, one of two scenarios is possible:
· Each step in the software engineering process should produce a deliverable that can be reviewed and that can act as a foundation for the steps that follow.
· The Software Project Plan is produced at the culmination of the planning tasks.
· It provides baseline cost and scheduling information that will be used throughout the software process.
· The Software Project Plan must
1. communicate scope and resources to software management, technical staff, and the customer
2. define risks and suggest risk aversion techniques
3. define cost and schedule for management review
4. provide an overall approach to software development for all people associated with the project
5. outline how quality will be ensured and change will be managed.