下面这段解释挺清楚的：

Approximately 80 percent of the students enrolled in U.S. medical schools receive some type of financial aid. Many, but not
all, forms of financial aid are need-based. This assumes that the student, spouse, or in some case the parents will pay for the
cost of education to the extent that they are able and that financial aid will be used to supplement the effort the family is
making.

“Need analysis” is the method used by financial aid officers to determine how much the student (and spouse) and
family can contribute to the cost of education.　There are two need analysis formulas in use.

“Federal Methodology” (FM) must be used to determine a student’s eligibility
for federal aid, primarily the subsidized Stafford loan. Federal aid eligibility is determined from financial data provided by
the student (and spouse) on the Free Application for Federal Student Aid (FAFSA). Because the FAFSA classifies all graduate
and professional students as independent, parental information is not part of FM for these students. Using FAFSA data, FM
calculates the student’s “expected family contribution” (EFC), which is displayed on the Student Aid Report (SAR) received
by the student. Financial need according to FM is the result of the cost of attendance minus the EFC.


The second need analysis formula is generally called “Institutional Methodology” (IM). This methodology determines the
parents’ ability to contribute to the cost of a medical education. The main components of the IM formula are: income,
assets, the number of family members, and the number of family members currently enrolled in college. IM also determines
the ability of the student (and spouse) to contribute to the cost of education. In doing this, IM includes some forms of
income and assets ignored by FM. Unlike FM, IM will vary in its particulars from school to school, and medical schools will
use different forms to collect parent and student (and spouse) data for their IM calculation.

A financial aid officer will use IM to award what is usually referred to as institutional financial aid. This aid includes both
scholarships and low interest loans, which come from various sources, the medical school, the Department of Health and
Human Services or private foundations. These funds are limited in amount and must be rationed more strictly than the aid
awarded according to FM. Financial need according to IM is the result of the cost of attendance minus the calculated parent
and student (and spouse) contributions.

Financial aid officers will follow both FM and IM in awarding need based aid. Students can always request reconsideration of
their awards if errors have been made in reporting or calculating data or if family circumstances change.
A third type of financial aid available to medical students is non-need-based aid. This type includes various unsubsidized
loans that can be used to replace both parent and student contributions as determined by FM and IM. Financial aid officers
must certify these loans because non-need-based loans cannot exceed the cost of attendance minus any need based aid
received by the student.


摘自：
http://www.aamc.org/students/financing/md2/phase2/pdf/phase2.i.pdf