Congratulations on your interview offers! I cannot remember exactly how long the interview was, but I was interviewed by two people. The interview format is standard, not a multiple mini interview. Purdue is looking for about 40 out-of-state students. My initial class size was 84, and it seemed like half of us were out-of-state students. I do not know/remember how many people were offered interviews. With regards to what degree the interview affects acceptance, I think the best mindset to have is that everyone that has been offered an interview is on the same level. In other words, after you have been offered an interview, your acceptance then depends on your interview (don’t let this make you nervous!). Many of the schools I interviewed with explicitly said this (again, I cannot remember if Purdue was one).
The interview itself was actually quite enjoyable. You will be told many times that your interviewers “just want to have a conversation with you” and I think that was very accurate. I was not asked any specific questions about my resume or experiences. I can’t remember many of the questions from the interview (partly due to the fact that it was really just a conversation). I believe I was asked what my biggest weakness is. I have no idea if you will be asked this but I would encourage you to have an answer that is unique to you! They truly will just be trying to get to know you, so try to relax and have fun with it!
I suppose I should add that many of classmates’ interviews were not as enjoyable for them as it was for me. Some of the faculty are less smiley/outgoing than others, but don’t let this deter you.
The last thing I would add (and I know this is easier said than done) is to be yourself in the interview. Your interviewers are not looking for certain qualifications, personalities, or experiences. My class is incredibly diverse, and if there is a rubric that the admission faculty are using, it’s hard to see what criteria they are looking at. Good luck!