Variability is important (the pesticide paradox)

There is no silver bullet in software testing. In other words, there is no single testing technique that you can always apply to find all possible bugs. Different testing techniques help reveal different bugs. If you use only a single technique, you may find all the bugs you can with that technique and no more.

A more concrete example is a team that relies solely on unit testing techniques. The team may find all the bugs that can be captured at the unit test level, but they may miss bugs that only occur at the integration level.

This is known as the pesticide paradox: every method you use to prevent or find bugs leaves a residue of subtler bugs against which those methods are ineffectual. Testers must use different testing strategies to minimize the number of bugs left in the software. When studying the various testing strategies presented keep in mind that combining them all is probably a wise decision.