First, Yale's martriculation rate is close to 70% in the late years (67-69%). This is extremely high in Ivies and only second to H;
Second, Yale did not accpet >50%. Yale accpet about 1900 per year with about 750 from EA;
Third, Yale's EA yield is close to 90% and overall close to 70% meaning yield of about 50% for RD applicants. This is very high.
If you google YDN and Yale admission rate or yield rate you will find plenty of info:
http://www.yaledailynews.com/news/2010/jan/11/early-admit-rate-stays-steady/
http://ivysuccess.com/yale_2013.html
http://www.yaledailynews.com/news/2010/apr/02/admit-rate-stays-flat-waitlist-swells/
http://www.yaledailynews.com/news/2010/may/27/yield-rate-sees-slight-decline/
http://www.yaledailynews.com/news/2009/may/08/yield-rate-beats-expectations/
First