This section is dedicated to the proposition that you can test best when you know as much as possible about your enemy: errors.
Which Classes Contain the Most Errors?
It's natural to assume that defects are distributed evenly throughout your source code. If you have an average of 10 defects per 1000 lines of code, you might assume that you'll have one defect in a class that contains 100 lines of code. This is a natural assumption, but it's wrong.
Capers Jones reported that a focused quality-improvement program at IBM identified 31 of 425 classes in the IMS system as error-prone. The 31 classes were repaired or completely redeveloped, ...