Software cost estimation  Predicting the resources required for

Software cost estimation Predicting the resources required for a software development process Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 1

Fundamental estimation questions How much effort is required to complete an activity? How much calendar time is needed to complete an activity? What is the total cost of an activity? Project estimation and scheduling and interleaved management activities Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 2

Software cost components Hardware and software costs Travel and training costs Effort costs (the dominant factor in most projects) salaries of engineers involved in the project Social and insurance costs Effort costs must take overheads into account Ian Sommerville 2000 costs of building, heating, lighting costs of networking and communications costs of shared facilities (e.g library, staff restaurant, etc.) Software Engineering, 6th edition. Chapter 23 Slide 3

Costing and pricing Estimates are made to discover the cost, to the developer, of producing a software system There is not a simple relationship between the development cost and the price charged to the customer Broader organisational, economic, political and business considerations influence the price charged Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 4

Programmer productivity A measure of the rate at which individual engineers involved in software development produce software and associated documentation Not quality-oriented although quality assurance is a factor in productivity assessment Essentially, we want to measure useful functionality produced per time unit Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 5

Productivity measures Size related measures based on some output from the software process. This may be lines of delivered source code, object code instructions, etc. Function-related measures based on an estimate of the functionality of the delivered software. Function-points are the best known of this type of measure Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 6

Measurement problems Estimating the size of the measure Estimating the total number of programmer months which have elapsed Estimating contractor productivity (e.g. documentation team) and incorporating this estimate in overall estimate Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 7

Lines of code What's a line of code? The measure was first proposed when programs were typed on cards with one line per card How does this correspond to statements as in Java which can span several lines or where there can be several statements on one line What programs should be counted as part of the system? Assumes linear relationship between system size and volume of documentation Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 8

Productivity comparisons The lower level the language, the more productive the programmer The same functionality takes more code to implement in a lower-level language than in a high-level language The more verbose the programmer, the higher the productivity Ian Sommerville 2000 Measures of productivity based on lines of code suggest that programmers who write verbose code are more productive than programmers who write compact code Software Engineering, 6th edition. Chapter 23 Slide 9

High and low level languages Low-level language Analysis Design C oding Validation High-le vel language Analysis Ian Sommerville 2000 Design Coding Validation Software Engineering, 6th edition. Chapter 23 Slide 10

System development times Assembly code High-level language Assembly code High-level language Ian Sommerville 2000 Analysis Design Coding Testing Documentation 3 weeks 5 weeks 8 weeks 10 weeks 2 weeks 3 weeks 5 weeks 8 weeks 6 weeks 2 weeks Size Effort Productivity 5000 lines 28 weeks 714 lines/month 1500 lines 20 weeks 300 lines/month Software Engineering, 6th edition. Chapter 23 Slide 11

Function points Based on a combination of program characteristics external inputs and outputs user interactions external interfaces files used by the system A weight is associated with each of these The function point count is computed by multiplying each raw count by the weight and summing all values Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 12

Function points Function point count modified by complexity of the project FPs can be used to estimate LOC depending on the average number of LOC per FP for a given language LOC AVC * number of function points AVC is a language-dependent factor varying from 200-300 for assemble language to 2-40 for a 4GL FPs are very subjective. They depend on the estimator. Ian Sommerville 2000 Automatic function-point counting is impossible Software Engineering, 6th edition. Chapter 23 Slide 13

Object points Object points are an alternative function-related measure to function points when 4Gls or similar languages are used for development Object points are NOT the same as object classes The number of object points in a program is a weighted estimate of Ian Sommerville 2000 The number of separate screens that are displayed The number of reports that are produced by the system The number of 3GL modules that must be developed to supplement the 4GL code Software Engineering, 6th edition. Chapter 23 Slide 14

Object point estimation Object points are easier to estimate from a specification than function points as they are simply concerned with screens, reports and 3GL modules They can therefore be estimated at an early point in the development process. At this stage, it is very difficult to estimate the number of lines of code in a system Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 15

Productivity estimates Real-time embedded systems, 40-160 LOC/P-month Systems programs , 150-400 LOC/P-month Commercial applications, 200-800 LOC/P-month In object points, productivity has been measured between 4 and 50 object points/month depending on tool support and developer capability Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 16

Quality and productivity All metrics based on volume/unit time are flawed because they do not take quality into account Productivity may generally be increased at the cost of quality It is not clear how productivity/quality metrics are related If change is constant then an approach based on counting lines of code is not meaningful Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 17

Estimation techniques There is no simple way to make an accurate estimate of the effort required to develop a software system Initial estimates are based on inadequate information in a user requirements definition The software may run on unfamiliar computers or use new technology The people in the project may be unknown Project cost estimates may be self-fulfilling Ian Sommerville 2000 The estimate defines the budget and the product is adjusted to meet the budget Software Engineering, 6th edition. Chapter 23 Slide 18

Estimation techniques Algorithmic cost modelling Expert judgement Estimation by analogy Parkinson's Law Pricing to win Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 19

Algorithmic code modelling A formulaic approach based on historical cost information and which is generally based on the size of the software Discussed later in this chapter Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 20

Expert judgement One or more experts in both software development and the application domain use their experience to predict software costs. Process iterates until some consensus is reached. Advantages: Relatively cheap estimation method. Can be accurate if experts have direct experience of similar systems Disadvantages: Very inaccurate if there are no experts! Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 21

Estimation by analogy The cost of a project is computed by comparing the project to a similar project in the same application domain Advantages: Accurate if project data available Disadvantages: Impossible if no comparable project has been tackled. Needs systematically maintained cost database Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 22

Parkinson's Law The project costs whatever resources are available Advantages: No overspend Disadvantages: System is usually unfinished Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 23

Top-down and bottom-up estimation Any of these approaches may be used top-down or bottom-up Top-down Start at the system level and assess the overall system functionality and how this is delivered through sub-systems Bottom-up Ian Sommerville 2000 Start at the component level and estimate the effort required for each component. Add these efforts to reach a final estimate Software Engineering, 6th edition. Chapter 23 Slide 24

Top-down estimation Usable without knowledge of the system architecture and the components that might be part of the system Takes into account costs such as integration, configuration management and documentation Can underestimate the cost of solving difficult low-level technical problems Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 25

Bottom-up estimation Usable when the architecture of the system is known and components identified Accurate method if the system has been designed in detail May underestimate costs of system level activities such as integration and documentation Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 26

Estimation methods Each method has strengths and weaknesses Estimation should be based on several methods If these do not return approximately the same result, there is insufficient information available Some action should be taken to find out more in order to make more accurate estimates Pricing to win is sometimes the only applicable method Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 27

Experience-based estimates Estimating is primarily experience-based However, new methods and technologies may make estimating based on experience inaccurate Ian Sommerville 2000 Object oriented rather than function-oriented development Client-server systems rather than mainframe systems Off the shelf components Component-based software engineering CASE tools and program generators Software Engineering, 6th edition. Chapter 23 Slide 28

Algorithmic cost modelling Cost 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 Most commonly used product attribute for cost estimation is code size Most models are basically similar but with different values for A, B and M Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 29

Estimation accuracy The 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 Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 30

Estimate uncertainty 4x 2x x Feasibility Requirements Design Code Delivery 0.5x 0.25x Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 31

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. Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 32

COCOMO 81 Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 33

COCOMO 2 levels COCOMO 2 is a 3 level model that allows increasingly detailed estimates to be prepared as development progresses Early prototyping level Early design level Estimates based on object points and a simple formula is used for effort estimation Estimates based on function points that are then translated to LOC Post-architecture level Estimates based on lines of source code Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 34

Early prototyping level Supports prototyping projects and projects where there is extensive reuse Based on standard estimates of developer productivity in object points/month Takes CASE tool use into account Formula is PM ( NOP (1 - %reuse/100 ) ) / PROD PM is the effort in person-months, NOP is the number of object points and PROD is the productivity Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 35

Object point productivity Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 36

Early design level Estimates can be made after the requirements have been agreed Based on standard formula for algorithmic models PM A SizeB M PMm where M PERS RCPX RUSE PDIF PREX FCIL SCED PMm (ASLOC (AT/100)) / ATPROD Ian Sommerville 2000 A 2.5 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 Software Engineering, 6th edition. Chapter 23 Slide 37

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 PM reflects the amount of automatically generated code Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 38

Post-architecture level Uses same formula as early design estimates Estimate of size is adjusted to take into account Requirements volatility. Rework required to support change Extent of possible reuse. Reuse is non-linear and has associated costs so this is not a simple reduction in LOC ESLOC ASLOC (AA SU 0.4DM 0.3CM 0.3IM)/100 » ESLOC is equivalent number of lines of new code. ASLOC is the number of lines of reusable code which must be modified, DM is the percentage of design modified, CM is the percentage of the code that is modified , IM is the percentage of the original integration effort required for integrating the reused software. » SU is a factor based on the cost of software understanding, AA is a factor which reflects the initial assessment costs of deciding if software may be reused. Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 39

The exponent term This depends on 5 scale factors (see next slide). Their sum/100 is added to 1.01 Example 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 Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 40

Exponent scale factors Scale factor Precedentedness Development flexibility Architecture/risk resolution Team cohesion Process maturity Ian Sommerville 2000 Explanation Reflects the previous experience of the organisation with this type of project. Very low means no previous experience, Extra high means that the organisation is completely familiar with this application domain. Reflects the degree of flexibility in the development process. Very low means a prescribed process is used; Extra high means that the client only sets general goals. Reflects the extent of risk analysis carried out. Very low means little analysis, Extra high means a complete a thorough risk analysis. Reflects how well the development team know each other and work together. Very low means very diffi cult interactions, Extra high means an integrated and effective team with no communication problems. Reflects the process maturity of the organisation. The computation of this value depends on the CMM Maturity Questionnaire but an estimate can be achieved by subtracting the CMM process maturity level from 5. Software Engineering, 6th edition. Chapter 23 Slide 41

Multipliers Product attributes Computer attributes constraints imposed on the software by the hardware platform Personnel attributes concerned with required characteristics of the software product being developed multipliers that take the experience and capabilities of the people working on the project into account. Project attributes Ian Sommerville 2000 concerned with the particular characteristics of the software development project Software Engineering, 6th edition. Chapter 23 Slide 42

Project cost drivers Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 43

Effects of cost drivers Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 44

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 1 Cost components Ian Sommerville 2000 Target hardware Development platform Effort required Software Engineering, 6th edition. Chapter 23 Slide 45

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 Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 46

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 Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 23 Slide 47