Hello Sudipta
All the IIMs usually follow a two-step selection process. First, short-listing on the basis of CAT score and then WAT and PI rounds for the short-listed students.
Common Admission Test (CAT)
Written Ability Test (WAT) and Personal Interview (PI)
Candidates who fulfil the first stage of the admission criteria (CAT) are called for WAT and PI rounds. Different IIMs as per their admission criteria offer admissions to the selected students.
Refer the below link for more information
https://bschool.careers360.com/articles/iim-admission-criteria
Regards
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