having automated tests primarily created and maintained either by QA or an outsourced party is not correlated with IT performance. The theory behind this is that when developers are involved in creating and maintaining acceptance tests, there are two important effects. First, the code becomes more testable when developers write tests. This is one of the main reasons why test-driven development (TDD) is an important practice—it forces developers to create more testable designs. Second, when developers are responsible for the automated tests, they care more about them and will invest more effort
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