Bugs / Source Code
This at least is how it always feels: a mystical 10% of the code base causing 90% of the reported bugs. This makes it very important to identify the 10% of the code that is causing the bugs and then resolve those issues as a matter of priority. Wasting effort on the other 90% of the code base until the problem 10% is resolved will not make a noticible improvement in the product quality.
The cost of bug fixing
The following graph is why I think the cost of additional design reviews and additional testing during implementation (e.g, unit testing and code reviews) is justified, because it is still far cheaper than fixing the issues later during testing or after product release (maintenance).
Source data from IBM Research.
