The Common Data Set, Page 7, shows NU’s official ranking of where legacy is to NU admissions - and it’s really not that important. GPA, test scores, ECs and essays are all more important than legacy to getting admitted. There is no mention of “NU gives preferences to applicants that are full pay”, and I had 2 kids admitted that were NOT full pay. BTW - I went to a MAC school - not an Ivy! 
http://enrollment.northwestern.edu/pdf/common-data/2014-15.pdf
In my school, the kid that was top 5% NMS Prez scholar but needed substantial financial aid (first generation) got denied at NU and accepted at Yale full scholarship and my friend who was none of the above, lower GPA and SAT almost no ECs but Dad went to an ivy and was full pay was accepted.So yes there is no mention of “NU gives preferences to applicants that are full pay” and yes you have familial evidence that suggests otherwise but I have seen this happen at my school.
@aurora2016 - anytime you apply to a top school it’s a crap shoot, and with so many well-qualified applicants for few openings, it’s always hard to understand admission decisions. I know that there were other kids in my kid’s classes & social groups that applied to NU and were not admitted.
IMHO, without a look at the specific files, nobody that is not in the admissions office will ever really understand the reasoning for final decisions. I’m sorry you were not admitted, but life goes on, as it did for my kid’s friends. There are a lot of happy, well educated kids that do NOT go to NU! Good luck to you.
@aurora2016, the plural of anecdote is not data. A stats class may be helpful. And what @nugraddad said.
Thanks for all your advice and wisdom. NU would be proud of your representation on these boards. Go wildcats! Yes my life has gone on and quite nicely. I am a junior at Yale. So no saltiness here…I stand by my opinions whether statistically accurate or not. But thanks for the wise words
Thanks again PurpleTitan and nugraddad