roulishollandais
Testing must be done by the team who elaborated the software. It is nearly impossible to debug a software yourself were not the conceiver.
I would disagree with this as most major 'software' problems are not faults in the code but faults in the design due to poor or incorrect systems analysis. As a simple case, if the systems analyst thought that a wind of 360 at 10kts is blowing
to the North not
from the North and designed the software with that in mind, the
software could be tested repeatedly and the fault would not be found. This is called
verification testing all it does is confirm that the software does what it was designed to do without errors.
Design faults though are only be found in
Validation testing where
the user confirms that all the functional requirements of the system perform correctly. The user tests the system as a black box - i.e. has and needs no knowledge of the design or the software. This kind of design fault through misunderstanding would only be found in validation testing. These design faults are also the most expensive faults to fix as the system is usually close to complete when validation testing is carried out and fixes will require a large amount of repeat (regression) testing to ensure the fix has not broken anything else.
Unfortunately, while it was seen as exceptionally important in the early days of computing, systems analysis and design has been somewhat trivialized in recent years into a semi-automated process and this has led to some very costly development failures.