It appears that Northwestern, unlike many schools, only defers a small number of ED applications.
The only recent info I could find doesn’t give a percentage, but just says that the “statistics for deferral are not as widely published as acceptance rates. According to the available information, many schools defer more than half of their early applicant pool to the regular admissions round. Notable exceptions include Duke, Middlebury, Northwestern, Notre Dame, and Stanford who deny most applicants who are not accepted in the early round. For these schools, deferral is used to indicate that your application is competitive and will be given serious consideration in the regular admissions process.”
http://www.collegiategateway.com/early-admissions-trends-for-the-class-of-2021/
From 2009, I found info saying that NU only started using deferrals at all in 2009, and that year deferred 60 students out of 1712 (about 3 1/2 %), with half of those offered admission in RD.
http://www.northbynorthwestern.com/story/northwestern-admissions-alters-early-decision-poli/
Does anyone know more recent numbers?
Thanks.