LizzyM in various threads for which legit quotes are locked because she is so overwhelmingly popular said:
You are new here so you may not have seen my analogy of the staircase. Your grades and scores (combined, if you will, with the formula GPA(10)+MCAT) along with your ECs, essays and LORs place you in one of many broad ranked categories. You can think of them as stairs on a wide staircase. (In other words, many of you can be on the same stair.) If a school selects you for interview, in all likelihood you are high enough on the stairs to be admitted or there is the potential that a good interview could boost you up enough to garner admission. That said, someone with a 4.0/40 and an amazing dossier of activities will start out on a higher stair and be more likely to be admitted if your interviews are about the same. Or, the other applicant could bomb the interview and move far down the staircase while you, with a great interview, move up. If a school looks at an applicant and says, "Even with a great interview, we couldn't possibly admit someone with an undergrad gpa of x.xx", then the school is doing you a disservice by inviting you to interview. If you get an interview, it should be a signal that you are "good enough" on paper and the next step is to determine if you are as good (or even better) in person.
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1. It isn't a ladder rung which suggests that only one person can occupy each rung. It is a wide staircase with many people on each stair.
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I've answered this before but here goes: It does vary by school. Imagine that people are standing on a huge staircase with those who have the highest stats and the most remarkable experiences at the top stair and downward to the least among those who are interviewed. After the interview, the applicants can keep their place on their original stair, go up a step or two or go down a step or many steps. In most cases, the people with the higest stats are still at the top but some are sent to the bottom step and some people move up or down according to their performance.
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My school doesn't really use numbers in this way but let me give you an example that roughly approximates what happens at one school.
Imagine a huge staircase with numbered stairs. On interview day, the applicants are on the stair that corresponds to their LizzyM score. An applicant that is very impressive on interview might be moved up one stair or more. Most applicants are going to remain where they are.. not going up or down. Some student who do or say something absolutely terrible might be sent down 10 steps, or more. In some cases, an applicant that didn't impress the interviewers but wasn't horrible might go down a step or three.
Now where is everyone? Starting at the top of the staircase, we admit students until we max out the number we can safely admit (without becoming oversubscribed). Obviously, scores and grades still matter but those who are great on interview can jump ahead and those who bomb will be demoted to the bottom of the waitlist or outright rejected.