Top-Down Estimation Approach
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Effort estimation is a process of approximating project costs and schedules, influenced by the amount of available information. Two main approaches are top-down and bottom-up estimation, where top-down begins with high-level components and descends to detailed ones. The COCOMO model is a widely used framework for estimating costs based on lines of code, using various cost drivers to adjust initial estimates for better accuracy.
Top-Down Estimation Approach
- 1. EFFORT ESTIMATION TOP-DOWN ESTIMATION APPROACH M. Sabiha 120419862042 MCA II Year
- 2. What is Effort Estimation? Effort Estimation is the process of finding an estimate, or approximation, which is a value that can be used for some purpose even if input data may be incomplete, uncertain, or unstable.
- 3. Why Effort Estimation? 1. Establishes the cost and schedule goals of the project 2. Determines the staffing level for a project during different phases 3. Depends on the level of information available about the project 4. The more detailed the information, the more accurate the estimation can be
- 5. Effort Estimation Approaches Top-Down Approach Bottom-Up Approach
- 6. •Also known as stepwise-refinement or decomposition, because this approach takes the whole software system as one entity and decomposes it to achieve more than one subsystem based on some characteristics. Top-Down Estimation Approach •This approach starts with designing the most basic components and proceeds to higher level components that uses these lower-level components. Bottom-Up Estimation Approach
- 7. Top Down Approach It considers the effort as the function of project size Where a and b are constants and the project size is generally in KLOC (thousands of lines of code). Values for these constants for an organization are determined through regression analysis, which is applied to data about the projects that have been performed in the past. EFFORT = a * SIZEb
- 8. COCOMO Model The Constructive Cost Model (COCOMO) is a procedural software cost estimation model based on LOC (Lines of code). In this model, for the initial estimate (also called nominal estimate) the equation for an organic project is E = 3.9(SIZE)0.91
- 9. Cost Drivers After determining the initial estimate, COCOMO uses a set of 15 different attributes of a project called cost driver attributes. Some of them are: Software Reliability Product Complexity Analyst Capability Application Experience Use of Modern Tools Required Development Schedule
- 10. EFFORT MULTIPLIERS FOR DIFFERENT COST DRIVERS
- 11. Cost Drivers Very Low Low Nominal High Very High Product Attributes RELY, required reliability DATA, database size CPLX, product complexity 0.75 0.70 0.88 0.94 0.85 1.00 1.00 1.00 1.15 1.08 1.15 1.40 1.16 1.30 Computer Attributes TIME, execution time constraint STOR, main storage constraint VITR, virtual machine volatility TURN, computer turnaround time 0.87 0.87 1.00 1.00 1.00 1.00 1.11 1.06 1.15 1.07 1.30 1.21 1.30 1.15 Personnel Attributes ACAP, analyst capability AEXP, application experience PCAP, programmer capability VEXP, virtual machine experience LEXP, programming lang. exp. 1.46 1.29 1.42 1.21 1.14 1.19 1.13 1.17 1.10 1.07 1.00 1.00 1.00 1.00 1.00 0.86 0.91 0.86 0.90 0.95 0.71 0.82 0.70 Project Attributes MODP, modern prog. practices TOOL, use of software tools SCHED, development schedule 1.24 1.24 1.23 1.10 1.10 1.08 1.00 1.00 1.00 0.91 0.91 1.04 0.82 0.83 1.10
- 12. Effort Adjustment Factor (EAF) If we multiply all the multiple factors of 15 different cost driver attributes, we get the Effort Adjustment Factor. To better understand EAF, let’s consider an example of developing a software application for the Hospital Management
- 13. The modules involved in this application would be Login 100 LOC Payment 100 LOC Appointment Booking 400 LOC Selecting Time Schedule 400 LOC TOTAL 1000 LOC
- 14. Using COCOMO Model… Suppose the cost drivers involved in developing this application would be – High complexity (1.15) Nominal execution time constraint (1.00) Nominal application experience (1.00) Very high development schedule (1.10)
- 15. Then, the Effort Estimation Factor is The initial effort estimate can be calculated from the relevant equation as Using the EAF, the adjusted effort estimate is EAF = 1.15*1.00*1.00*1.10 = 1.265 Ei = 3.9 * 10.91 = 3.9 PM E = 1.265 * 3.9 = 4.93 PM
- 16. COCOMO’s Phase-wise Distribution of Effort Phase Small 2 KLOC Intermediate 8 KLOC Medium 32 KLOC Large 128 KLOC Product Design 16 16 16 16 Detailed Design 26 25 24 23 Code and unit test 42 40 38 36 Integration and test 16 19 22 25
- 17. Advantages of Top-Down Approach 1. This approach is more suitable when the software solution needs to be designed from scratch and specific details are unknown 2. In top-down approach, the heads of an organization make the decisions and pass them down to other members of the organization to implement. 3. It helps align the project goals with the organization’s strategic goals as upper management is giving the directions. 4. For top-down estimation to work well, it is important that good estimates for the size of the software be obtained.