(Heavily condensed from the original, which you should read in its entirety.)
More Isn’t Always Better: Higher code coverage was not the best measure of post-release failures in the field. If 99 percent of the code has been tested, but the 1 percent that did not get tested is what customers use the most, then there is a clear mismatch between usage and testing. It is more beneficial to achieve higher code coverage of more complex code than to test less complex code at an equivalent level. Those are the kinds of tradeoffs that development managers need to keep in mind.
Write Test Code First: TDD teams produced code that was 60 to 90 percent better in terms of defect density than non-TDD teams. They also discovered that TDD teams took longer to complete their projects--15 to 35 percent longer. “Over a development cycle of 12 months, 35 percent is another four months, which is huge. However, the tradeoff is that you reduce post-release maintenance costs significantly, since code quality is so much better. Again, these are decisions that managers have to make--where should they take the hit?”
Proving the Utility of Assertions: More assertions and code verifications means fewer bugs. Looking behind the straight statistical evidence, they also found a contextual variable: experience. Software engineers who were able to make productive use of assertions in their code base tended to be well-trained and experienced, a factor that contributed to the end results. What kind of action should development managers take based on these findings? The research team believes that enforcing the use of assertions would not work well; rather, there needs to be a culture of using assertions in order to produce the desired results.
Organizational Structure Does Matter – a Lot: Organizational metrics, which are not related to the code, can predict software failure-proneness with a precision and recall of 85 percent. This is a significantly higher precision than traditional metrics such as churn, complexity, or coverage that have been used until now to predict failure-proneness.
Geographical Distance Doesn’t Matter – Much: Most people preferred to talk to someone from their own organization 4,000 miles away rather than someone only five doors down the hall but from a different organization. Organizational cohesiveness played a bigger role than geographical distance.