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SDN article: Legal Matters: Joining a Board
All schools have a secondary. Some automatically send it to you if you submit a primary. Some will only send you a secondary if you meet some cutoff in the primary applications. Some of the secondaries are as simply as a couple of checkboxes. Some involve multiple essays. Just go with the flow and make a little spreadsheet to keep track!
Actually, according to the MSAR 2009-2010, there are five schools that don't have a secondary application. They are Mayo, UMDNJ-RWJMS, UTMB, UT-Houston, UT-San Antonio. Unless you are applying to these schools, expect a secondary. Plus, there are some schools that will screen you prior to your secondary so the ones that they will formally discuss are those who only get the invitation to fill out the secondary.