Chapter 5. Defects and Design

Unfortunately, no programmer is perfect. Good programmers will introduce roughly one defect into a program for every 100 lines of code they write. The best programmers, under the best possible circumstances, will introduce one defect per 1,000 lines of code they write.

In other words, no matter how good or bad you are as a programmer, it’s certain that the more you code, the more defects you will introduce. This allows us to state a law called the Law of Defect Probability:

  • The chance of introducing a defect into your program is proportional to the size of the changes you make to it.

This is important because defects violate our purpose of helping people, and therefore should be avoided. Also, fixing defects is a form of maintenance. Thus, increasing the number of defects increases our effort of maintenance.

With this law, without having to predict the future, we can immediately see that making small changes is likely to lead to lower maintenance effort than making large changes would. Small changes = fewer defects = less maintenance.

This law is also sometimes stated more informally as “You can’t introduce new bugs if you don’t add or modify code.”

The funny thing about this law is that it seems to be in conflict with the Law of Change—your software has to change, but changing it will introduce defects. That is a real conflict, and it’s balancing these laws that requires your intelligence as a software designer. It is actually that conflict that explains ...

Get Code Simplicity now with the O’Reilly learning platform.

O’Reilly members experience live online training, plus books, videos, and digital content from nearly 200 publishers.