Rupalli,
It depends on the processes used in the performing organization.
In company A the feasibility study can be performed as an initial step before you even begin a project. Here the intention is to determine if the project has any merit. So the feasibility study becomes an input to the project selection criteria.
In company B, however, the feasibility study is the first phase of a project. Here the intention is take the original project idea and figure out which of the many options that we have is the one that would be most successful. The study is used to determine (for instance) the best scope of the project.
Ans lastly in company C, the feasibility study study is treated as a project of it's own. (In my previous job in Switzerland we called this a "Vorprojekt" - a "pre project".) You may be running several such studies simultaneously and then use the results to determine which projects you want to take on.
To my knowledge there really are no best practices in regards to what the exact contents and the specific outputs of a feasibility study should be.
Until Next Time,
Cornelius Fichtner, PMP, CSM
President, OSP International LLC