---
title: "Why software reliability matters"
subject: "Software Engineering"
chapter: 9
key: "semester_5/software_engineering:reliability-importance"
source: https://www.bctengineer.com/q/semester_5/software_engineering/reliability-importance
site: BCT Engineering
---

# Why software reliability matters

Software Engineering · Chapter 9: Quality management

- Asked in: year not given, 4 marks
- Video: [L53: Software Reliability Introduction | Types of Time in Software Engineering Lectures(Course)](https://www.youtube.com/watch?v=6_L6HNeDRCU) — Easy Engineering Classes
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/software_engineering/reliability-importance
- Whole chapter: https://www.bctengineer.com/md/semester_5/software_engineering/chapter-9

## Question

Explain why software reliability is important.

## Short answer

Software reliability is the probability of failure-free operation; it is important because users expect it, failures are costly, data may be lost, safety-critical failures can kill, and reputation/legal liability suffer. It is often more important than efficiency and hard to add later.

## Answer

**Software reliability** is the probability that software operates without failure in a specified environment for a specified time. It is one of the most important quality attributes, often more important than efficiency.

**Why reliability is important** (Sommerville):
1. **Users expect it:** people rely on software in daily life (banking, mobile payment, booking). Users will not trust or use a system that fails often, even if it has many features.
2. **Cost of failure is very high:** a failure can cost much more than the system itself. For example, a failure in a bank's core system can stop all transactions for hours.
3. **Unreliable systems may lose information:** data is often worth more than the software that processes it. Loss or corruption of customer records is expensive to recover from.
4. **Safety-critical systems:** in medical, aviation, railway and nuclear systems, a failure can cause injury or death. The Therac-25 radiation machine overdosed patients because of software faults.
5. **Business reputation and legal liability:** repeated failures damage the developer's reputation and can lead to lawsuits and penalties.
6. **Hard to add later:** unreliable software is expensive to fix after delivery, because faults are found late and every fix risks new faults.

**Reliability vs. efficiency:** computers are fast and cheap, so a slower program is usually acceptable, but an unreliable one is not. Inefficiency can be improved by better hardware, while unreliability cannot.

**Example:** the Ariane 5 rocket was destroyed in 1996 because of an unhandled numeric overflow in reused software, causing a loss of hundreds of millions of dollars.

For these reasons reliability requirements (such as MTTF or availability) are specified, measured during testing, and used to decide when the software is ready for release.

## Check yourself

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

1. Software reliability is:
   A. The speed at which software runs
   B. The number of features it has
   C. The probability of failure-free operation in a specified environment for a specified time
   D. The size of the code
   Answer: C. Reliability is the probability that software operates without failure in a specified environment for a specified time. (Key point: Definition of reliability)

2. Why is reliability often more important than efficiency?
   A. Inefficiency can be improved by better hardware, but unreliability cannot
   B. Efficient programs are always unreliable
   C. Hardware is expensive and cannot be upgraded
   D. Users never care about speed
   Answer: A. Computers are fast and cheap so slowness is tolerable, while unreliability cannot be fixed by hardware. (Key point: Reliability vs efficiency)

3. Which example shows the danger of unreliable software in safety-critical systems?
   A. Slow loading of a web page
   B. Therac-25 radiation machine overdosing patients
   C. A typo in a user manual
   D. A late software release
   Answer: B. Therac-25 overdosed patients because of software faults, showing failures can cause injury or death. (Key point: Safety-critical failures)

4. Why is data loss from unreliable systems serious?
   A. Data is rarely valuable
   B. Data can always be recreated instantly
   C. Only the software is worth money
   D. Data is often worth more than the software and is expensive to recover
   Answer: D. Data is often worth more than the software processing it, and recovery is costly. (Key point: Cost of failure)

## Related in Software Engineering

- [Software reliability and reliability models](https://www.bctengineer.com/question.md?key=semester_5%2Fsoftware_engineering%3Areliability-models) — Software Engineering, chapter 9, asked 2×

## Related in other subjects

- [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
- [Increasing reliability of embedded real-time software](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aembedded-rt-reliability) — Instrumentation II, chapter 9
- [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