When to start Testing in software testing process?

Hi,

Do you know when to start testing of application?
Some body tell that to start testing when developer build application.
Some body tell that to start testing when developer build any unit or module of application.
some one say that as early as possible.
But actually as per my knowledge, it should start during requirement gathering.
We have to start software testing when we start requirement gathering of system

Why to start software testing during requirement gathering?

Reason for that is if you start testing during requirement gathering then you can decrease the possibility of bug before system or unit development.

Then again one question How?
if you have referred requirement document then you can find many points which are actually not required in system or not useful in system. because client don't think about all points when he provide his requirement. you can find missing, incomplete or very requirement in Software requirement specification document.
If you have found all these defects during requirement it is positive. because you have prevented system from bug/defect.
one thing more that in SDLC's different phases, requirement gathering is first phase.
In SDLC, if any bug/defect found during system testing, integration testing or acceptance testing are more cost affective then bug/defect found during requirement gathering phase.
so this is reason of why to start testing during requirement gathering.

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