The question of whether or not Northwestern gives preferential treatment to local applicants came up in another thread. There are really two parts to this:
1 - Does Northwestern give preferential admissions to locals? If so, is it only in the ED round or in the RD round as well?
2 - What counts as local? Evanston, City of Chicago, North Shore, suburban Chicago or all of Illinois?
IMO - No, NU does not favor locals. NU has a national presence and rep - they have students from almost all 50 states, and a lot of internationals too. The NU Common Data set indicates that place of residence has no bearing on admissions: http://enrollment.northwestern.edu/pdf/common-data/2014-15.pdf
ClarinetDad126 - Nice, but the link is only scholarship info for local kids - it does not mention an admission preference, which is what the OP asked about.
This source estimates that 70% of NU students are from out of state:
http://www.collegefactual.com/colleges/northwestern-university/student-life/diversity/