Defect Clustering in the world of Testing

Definition :-

Defect Clustering refers to congregating of bugs in a small module of an application which shows operational failures.

Overview :-

Defects tend to assemble in a particular module because of the complexity of the code, here, in this case, the running the same test cases won't help us resolve the issue because they shall be providing with the same result time and again.

Phenomenon on which Defect Clustering is based: - Pareto Principle 

Pareto Principle: - Maximum things in life (effort, reward, output) are not distributed evenly, some contribute more than others.
It applies 80:20 rule, which means that 80% of the issues are the result of 20% modules of an application.

How is it useful: -

Thanks to this approach the tester focuses more on a particular module more because a couple of more bugs might arise in the same module due to iterations made while fixing some other bug which in turn would help the tester test the application thoroughly.

After a couple of iterations being made during bug fixing, the no. of defects found are reduced to a certain extent and the "HotSpot" module is left with no bugs.In order to be more confident about the module, we should run test cases.

We left with two approaches.

1.Either we write a new set of test cases in order to test different parts of the application.
2.Or to write new test cases & add them to the existing one.

But this nowhere means that the tester should  not test the entire application.
This phenomenon would only ensure that more regression testing is performed in that particular module.