I was too late too to add this comment to the above.
During peak reading season, every college employs external readers, temporary workers who are trained to read applications and rate them according to institutional requirements. A college like Harvard could employ another 40 external readers to help them get through all the applications. If you want to logistically calculate the process, it would be something like
20 Admissions staff + 40 external readers = 60 people reading applications
60 people times 20 applications per-person, per day = 1200 applications per-day
1200 applications per-day times 30-days (6 weeks) = 36,000 applications read
It’s a lot of hard work, but it’s very manageable for the Regional Admissions Committees, with outside help, to read every RD application between mid-January and the end of February. Then in March, the Regional Admissions Committees bring their strongest candidates to the full Admissions Committee for review and approval.
Although this article is not about Harvard, it gives the reader somewhat of an understanding of the holistic applications process by an outside reader: http://www.nytimes.com/2013/08/04/education/edlife/lifting-the-veil-on-the-holistic-process-at-the-university-of-california-berkeley.html?hp