Chapter 1. The promise of good tests
Figure 1.1. The more tests we have, the less value an additional test is likely to yield. The first tests we write most likely target some of the most crucial areas of the code base and as such tend to be high-value and high-risk. When we’ve written tests for almost everything, most likely the bits that are still lacking test coverage are the least crucial and least likely to break.
Figure 1.2. The first plateau lies where writing more tests or better tests no longer yields additional value. The second plateau is further up the hill, where we uncover further payoffs from changing our mindset—thinking about tests and testing as a resource richer than a mere tool for verification.