《代码大全》第20章，讨论软件质量。

平时工作经常被组里服务的软件质量困扰，觉得离 App 稍远的部分，被过度轻视，离 App 近的又一味依赖测试，总的质量管理其实很脆弱，开发的成本也危楼高百尺。读这部分，在主观感觉之外，找到了很多研究印证，有些数据和结论也比较有意思。遂记录。

External and internal quality characteristics

External

• Correctness
• Usability
• Efficiency
• Reliability
• Integrity??
• Adaptability
• Accuracy
• Robustness

Internal

• Maintainability
• Flexibility
• Portability
• Reusability
• Readability
• Testability
• Understandability

Techniques for detecting defects

结论:

缺陷检测方法组合使用的效果优于单独使用。

Defect-detection methods work better in combination than they do singly.

研究发现：

• 典型的组织使用重度测试的缺陷检测方法，只能达到大约85%的缺陷移除效率。领先的组织使用更广泛的技术，可以达到95%或更高的缺陷移除效率。

The typical organization uses a test-heavy defect-removal approach and achieves only about 85 percent defect-removal efficiency. Leading organizations uses a wider variety of techniques and achieve defect-removal efficiencies of 95 percent or higher.

• 人工过程（如检查和走查）比计算机测试更擅长发现某些类型的错误，而另一些类型的错误则相反。

Human processes (inspections and walk-throughs, for instance) tend to be better than computer-based testing at finding certain kinds of errors and the opposite is true for other kinds of errors.

• 单独使用测试的效果不佳。单独使用单元测试、功能测试和系统测试，通常缺陷检测效率不到60%，这通常对于生产软件来说是不够的。

The outlook for the effective of testing used by itself is bleak." … “A combination of unit testing, functional testing, and system testing often results in a cumulative defect detection of less than 60 percent, which is usually inadequate for production software.

Cost of Finding Defects

结论:

大多数研究发现，审查比测试更便宜。

Most studies have found that inspections are cheaper than testing.

研究数据：

• 与测试相比，读代码每小时发现的错误数量多 80%.

code reading detected about 80 percent more faults per hour than testing.

• 要发现软件设计中的缺陷，测试花费的时间是代码审查的6倍。

It costs six times as much to detect design defects by using testing as by using inspections.

• 3.5个工时的代码审查，可以发现一个错误，而测试需要15-25个工时才能发现一个错误。

Only 3.5 staff hours were needed to find each error when using code inspections, whereas 15-25 hours were needed to find each error through testing.

Cost of Fixing Defects

结论：

更早发现错误的检测技术能带来更低的修复成本。

A detection technique that finds the error earlier therefore results in a lower cost of fixing it.

研究数据：

• 代码审查是一种一步式的技术，而测试是一种两步式的技术。使用代码审查发现和修复一个缺陷需要3个小时，而使用测试发现和修复一个缺陷需要12个小时。

It takes three hours to find and fix a defect by using code inspection, a one-step technique, and 12 hours to find and fix a defect by using testing, a two-step technique.

• 一份关于超过400名开发者构建的70万行程序的报告表明，代码审查的成本效益是测试的数倍——前者投资回报率为1.38，而后者仅为0.17。

According to a report on “a 700,000-line program built by over 400 developers”, “Code reviews were several times as cost-effective as testing - a 1.38 return on investment vs. 0.17.

The General Principle of Software Quality

即：提高代码质量会降低开发成本。

• The General Principle of Software Quality is that improving quality reduces development costs.
• The single biggest activity on most projects is debugging and correcting code that doesn’t work properly.

研究数据：

• 166个程序员写代码解决同一个问题，最终平均代码量为220行，平均用时约5h。花费时间在中位数上下的程序员，代码中的 bug 最多。花费时间低于或高于中位数时间的程序员，代码 bug 明显较少。

结论：好的未必慢，差的未必快。