Software Cost Estimation: Techniques, Models & Example
Accurately estimating the cost and duration of a software project is one of the most critical and challenging tasks in software engineering. Before development begins, project managers must forecast how long the project will take and how much it will cost. But how is this achieved? The answer lies in software cost estimation techniques and models.
Why is Software Cost Estimation Important?
Before diving into development, it's essential to understand:
- Scope of the project
- Resources needed (people, tools, infrastructure)
- Development time required
- Budget planning
- Scheduling and team allocation
Cost estimation helps make informed decisions regarding staffing, timelines, progress tracking, and risk management.
Common Attributes of Estimation Procedures
All standard estimation techniques share these characteristics:
- The project scope is clearly defined before estimation begins.
- Software metrics (like Lines of Code) are used as reference points.
- The entire project is broken down into smaller parts, and each part is estimated separately.
- Estimation tools and models (manual or automated) are often used to enhance accuracy.
- Decomposition techniques help break down tasks and calculate associated costs and durations.
Uses of Cost Estimation
- Project Planning
2.Project Monitoring
During execution, estimation allows you to assess whether the project is on track and take corrective actions when necessary.
Cost Estimation Models
Estimation models can be classified as:
Static Models
- Based on one or more input variables.
- Variables are considered independent of each other.
Dynamic Models
- All variables are interconnected.
- Changes in one variable impact others.
Static, Single-Variable Models
These models use a single parameter (usually size of the software in Lines of Code - LOC) to estimate cost, effort, or duration.
A general formula:
C = a * L^b
Where:
- C = Cost
- L = Size (in LOC)
- a and b = Model-specific constants
Example: SEL Model (Software Engineering Laboratory)
Effort (E) = 1.4 * L^0.93
Documentation (DOC) = 30.4 * L^0.90
Duration (D) = 4.6 * L^0.26
Where:
- E = Effort in person-months
- DOC = Documentation pages
- D = Duration in months
- L = Lines of Code
Static, Multi-Variable Models
These models incorporate multiple variables (such as team experience, tool usage, complexity, etc.) to estimate effort and time.
Example: Walston-Felix (W-F) Model by IBM
Effort (E) = 5.2 * L^0.91
Duration (D) = 4.1 * L^0.36
Additionally, a Productivity Index can be calculated using:
Productivity = Σ (Wi * Xi)
Where:
- Wi = Weight factor for the i-th variable
- Xi = Estimator value {-1, 0, +1} indicating if the variable reduces, does not affect, or increases productivity
Example Comparison: SEL Model vs Walston-Felix Model
Let’s say a project requires 8 person-years of effort, i.e., 8 * 12 = 96 person-months.
(a) LOC Estimation
Using the inverse of the effort equations:
SEL Model:
L = (96 / 1.4)^(1 / 0.93) ≈ 94,264 LOC
W-F Model:
L = (96 / 5.2)^(1 / 0.91) ≈ 24,632 LOC
(b) Duration Estimation
SEL Model:
D = 4.6 * (94.264)^0.26 ≈ 15 months
W-F Model:
D = 4.1 * (24.632)^0.36 ≈ 13 months
(c) Productivity (LOC per Person-Year)
SEL Productivity:
Productivity = 94,264 LOC / 8 PY ≈ 11,783 LOC/PY
W-F Productivity:
Productivity = 24,632 LOC / 8 PY ≈ 3,079 LOC/PY
(d) Average Manning
Average manning is the average number of people working per month:
Average Manning = Effort (person-months) / Duration (months)
SEL Model:
96 / 15 ≈ 6.4 persons/month
W-F Model:
96 / 13 ≈ 7.4 persons/month
Final Thoughts
Software cost estimation is a vital part of project planning. Whether you're using a single-variable model or a more complex multivariable approach, the goal is the same: to deliver projects on time, within budget, and with the desired quality.
