The Personal Software Process Overview, Practice, and Results
Watts S. Humphrey
You would probably agree if I told you that the number of defects to be found in testing a program would be proportional to the number in the product when it entered test. It seems reasonable to find more defects when there are more to be found. If, however, I said that the number of defects in the product after test would be proportional to the number on test entry, many software engineers would not agree.