---
title: "Statistical SQA"
subject: "Software Engineering"
chapter: 9
key: "semester_5/software_engineering:statistical-sqa"
source: https://www.bctengineer.com/q/semester_5/software_engineering/statistical-sqa
site: BCT Engineering
---

# Statistical SQA

Software Engineering · Chapter 9: Quality management

- Asked in: year not given, 4 marks
- Video: [Software Reliability & Statistical Software Quality Assurance |SE|](https://www.youtube.com/watch?v=D2Y-2RO2fsQ) — Trouble- Free
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/software_engineering/statistical-sqa
- Whole chapter: https://www.bctengineer.com/md/semester_5/software_engineering/chapter-9

## Question

Explain statistical SQA.

## Short answer

Statistical SQA quantitatively analyzes defect data, traces errors to underlying causes, and applies the Pareto principle to find the vital few. It corrects these causes and remeasures; Six Sigma's DMAIC is a widely used implementation.

## Answer

**Statistical SQA** is a quantitative approach to software quality assurance. Data about software defects is collected and analysed, and the few causes that produce most of the defects are found and corrected.

**Steps** (Pressman):
1. **Collect and categorize** information about errors and defects from reviews, testing and field use.
2. **Trace each error to its underlying cause**, such as incomplete or erroneous specification (IES), misinterpretation of customer communication (MCC), error in data representation (EDR), violation of programming standards (VPS), or incomplete testing (IET).
3. **Apply the Pareto principle**: about 80% of defects come from about 20% of the causes. Isolate these "vital few" causes.
4. **Correct the problems** behind the vital few causes by improving the process, then keep measuring.

**Example**

| Cause | Defects | % |
|---|---|---|
| IES | 90 | 30 |
| MCC | 60 | 20 |
| EDR | 45 | 15 |
| 9 other causes | 105 | 35 |
| Total | 300 | 100 |

Three causes produce 65% of the defects. The team therefore introduces SRS templates and reviews (for IES), prototypes shown to the customer (for MCC), and data-structure reviews (for EDR), instead of spreading effort over all twelve causes.

**Six Sigma**, the most widely used statistical quality strategy, follows the same idea with its DMAIC steps: Define, Measure, Analyze, Improve, Control.

## Check yourself

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

1. Why does statistical SQA analyse defect data?
   A. To find the few causes that produce most defects and correct them
   B. To assign blame to individual programmers
   C. To count the total lines of code
   D. To replace all testing
   Answer: A. The aim is to locate and correct the few causes responsible for most defects. (Key point: Statistical SQA purpose)

2. A defect is found because the SRS omitted a requirement. Which cause category is it?
   A. EDR - error in data representation
   B. VPS - violation of programming standards
   C. MCC - misinterpretation of customer communication
   D. IES - incomplete or erroneous specification
   Answer: D. A missing requirement in the SRS is an incomplete or erroneous specification (IES). (Key point: Tracing errors to causes)

3. In the example, IES, MCC and EDR produce 65% of 300 defects. What does the team do?
   A. Spreads equal effort over all twelve causes
   B. Ignores them since defects are normal
   C. Targets these causes with SRS templates and reviews, prototypes and data-structure reviews
   D. Stops measuring defects
   Answer: C. Effort is focused on the vital few causes by changing the process, rather than spreading it over all causes. (Key point: Acting on vital few)

## Related in Software Engineering

- [SQA and statistical SQA](https://www.bctengineer.com/question.md?key=semester_5%2Fsoftware_engineering%3Asqa-statistical) — Software Engineering, chapter 9