Primary screeners are a mix of MS4s or higher (you basically need to have completed 3rd year and can be on a research year and still help with admissions) and faculty who have signed up to help with admissions for this cycle. Once the primary screener makes a decision on an application it then moves onto a secondary screener. The secondary screener's decision is the one that counts tbh. Secondary screeners include the dean of admissions and I think one or two other faculty members who have set aside a significant time to help with admissions. If I had to guess I'd say the dean of admissions probably does half of the secondary screens of all applications.
Sometimes faculty members who are helping for the first time realize the time it takes to help with screening, or interviewing and attending the committee meetings is far greater than they can afford to give. At the screening stage this may be one reason why an applicant's review can be somewhat delayed due to having to re-distribute it to another screener.
In terms of guidelines, we all have (even new faculty members) to go through orientation which is led by the dean of admissions as well as sign a code of conduct/ethics. During orientation, we discussed the various ways bias can influence our decisions and are reminded to keep our mindset on objective holistic review. At Sinai, MS1s and MS2s can help with screening for Sinai's FlexMed program only (early assurance). There's orientation for FlexMed screening too. So for those of us who have helped with admission going back to our MS1/MS2 days we definitely have more experience than the MS4 helping for the first time. Ultimately, the second screener's decision overpowers that of any primary screener. I've had the dean of admission give an interview for someone who I did not recommend an interview to and vice versa. I think admissions likes to have students help because 1. otherwise it would take forever to screen everybody and 2. as students we know the culture of Sinai very well and we are essentially deciding whether this person can fit in well here and would make a great classmate. Some of the best applications I've read made me go "Goodness, I wish this person was my classmate" or "This person embodies our mission at Sinai to the T." I think that's what most med schools base their decision on when handing out interviews.
