---
title: "Spiral model of software development"
subject: "Instrumentation II"
chapter: 9
key: "semester_5/instrumentation_ii:spiral-model"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/spiral-model
site: BCT Engineering
---

# Spiral model of software development

Instrumentation II · Chapter 9: Software For Instrumentation And Control Applications

- Asked in: 2070 Asar, 5 marks; 2076 Chaitra, 5 marks; 2075 Chaitra, 4 marks (asked 3 times)
- Video: [Spiral model | Software Engineering | SE | Lec-06 | Bhanu Priya](https://www.youtube.com/watch?v=YfGvIhPXz1A) — Education 4u
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/spiral-model
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-9

## Question

Spiral model of software development combines the elements of both waterfall and prototyping model. Explain the process of software development using spiral model.

## Short answer

The spiral model iteratively goes through objective setting, risk analysis, development, and review in each loop. It combines waterfall phases with prototyping and explicit risk handling, producing increasingly complete versions, suitable for large high-risk projects.

## Answer

The **spiral model** (Barry Boehm) is a risk-driven, iterative process. It combines the stepwise, controlled phases of the waterfall model with the iterative prototyping model, and adds explicit **risk analysis** at every loop. The project moves outward through a spiral; each loop is a phase (concept, requirements, design, implementation) and produces a more complete version.

```text
 +---------------------+---------------------+
 | 1 Objectives,       | 2 Risk analysis,    |
 |   alternatives      |   prototype         |
 +---------------------+---------------------+
 | 4 Plan next loop,   | 3 Develop and       |
 |   customer review   |   test              |
 +---------------------+---------------------+
   Loops go outward: concept -> requirements
   -> design -> code/test -> release
```

#### Process (each loop has four quadrants)
1. **Determine objectives, alternatives and constraints:** the requirements for this loop are set (performance, functionality, cost), the alternative solutions are listed.
2. **Evaluate alternatives and identify/resolve risks:** risks (technical, schedule, cost, requirement uncertainty) are analysed; prototypes, simulation or benchmarks are built to reduce them. If a risk is too high, the project may stop.
3. **Develop and verify the next level of product:** design, coding, and testing for the loop (e.g. concept of operation, then requirements, then design, then the code, then acceptance), using waterfall for that cycle.
4. **Plan the next iteration:** the customer reviews the result and the plan for the next loop (and budget) is prepared.

The first loops produce the concept and prototype; later loops produce the detailed design, code and tests, until the final system is delivered. The radius of the spiral represents cumulative cost, and the angle the progress.

**Advantages:** risk handled early, flexible for changes, customer feedback each loop, suits large and high-risk projects.
**Disadvantages:** needs risk-analysis expertise, costly, complex to manage, not suitable for small projects.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What distinguishes the spiral model?
   A. It is risk-driven, combining waterfall and prototyping, with risk analysis in every loop
   B. No iteration
   C. No customer review
   D. Only one phase
   Answer: A. Boehm's model adds explicit risk analysis to iterative loops. (Key point: Spiral origin)

2. What is done in the second quadrant of each loop?
   A. Customer review
   B. Evaluate alternatives and identify/resolve risks using prototypes, simulation or benchmarks
   C. Final deployment
   D. Code writing only
   Answer: B. Risks are analysed and reduced; if too high the project may stop. (Key point: Quadrant 2)

3. In which quadrant does the customer review and the next loop get planned?
   A. First
   B. Second
   C. Third
   D. Fourth
   Answer: D. Quadrant 4 is plan the next iteration with customer review. (Key point: Quadrant sequence)

4. What do the radius and angle of the spiral represent?
   A. Radius is progress, angle is cost
   B. Radius is cumulative cost, angle is progress
   C. Radius is time, angle is risk
   D. Radius is size, angle is team
   Answer: B. Radius shows cumulative cost; angle shows progress. (Key point: Spiral axes)

## Related in Instrumentation II

- [Software reliability; waterfall vs prototyping; embedded software](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Asw-reliability-models-embedded) — Instrumentation II, chapter 9
- [Prototyping model of software](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aprototype-model) — Instrumentation II, chapter 9, asked 3×
- [Software process models and effort estimation](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Asw-process-models) — Instrumentation II, chapter 9, asked 2×
- [Compare software models; propose a fourth model](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Asw-models-compare-fourth) — Instrumentation II, chapter 9
- [Shortcomings of software development models](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Asw-model-shortcomings) — Instrumentation II, chapter 9
- [Embedded real-time software; model for project](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aembedded-rt-software-model) — Instrumentation II, chapter 9

## Related in other subjects

- [Software runaways; waterfall and prototyping inside spiral](https://www.bctengineer.com/question.md?key=semester_5%2Fsoftware_engineering%3Asoftware-runaways-spiral) — Software Engineering, chapter 1
- [Waterfall versus spiral; user participation](https://www.bctengineer.com/question.md?key=semester_5%2Fsoftware_engineering%3Awaterfall-vs-spiral) — Software Engineering, chapter 1
- [Spiral model of embedded software life cycle](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aspiral-model-esdlc) — Embedded System, chapter 3
- [Current process models; why waterfall is not accurate](https://www.bctengineer.com/question.md?key=semester_5%2Fsoftware_engineering%3Awaterfall-not-accurate) — Software Engineering, chapter 1