Dear Sri Santosh;

Users' studies are done on purpose. There are various reasons  and they yield verious results too. Howeve, the main idea is to understand the specific TREND like say if you do user studies referencing patterns of Ph.D scholars in three different subjects like say Molecular Biology, History and Instrumentation Engineering and search 50 PhD. theses of students of each subjects, you will certainly find some different sets of patterns.

First among History students you will find plenty references of books & monographs and if you further concentrate on years of publications,  you will find that many old books are referred and in journals too you will find old references.

In Molecular Biology Theses, you will find references of mostly of journals and a few Conference Proceedings and those too of recent times. It is unlikely to find references beyond 7 years in case4 of Molecular Biology.

In Instrumentation Engineering, you may find references of patents in a fairly good percentage.

Now by this users' survey, you may deduce certain things

(A) Researchers in History cites books more than any other materials

(A.1) they often refer old materials

(A.2) Besides, books and periodicals they also refer to Reports  and treaties as well

(A.2) They often Indian references (of books &/or journals)

(B) The Molecular Biologists mostly refer Periodical articles and a small percentage of Conference papers

(B.1) By and large the referred articles are of recent times

(B.2) Majority of references are not old than 7 years

(B.3) Books do not constitute as major source of reference in Molecular Biology Research

(B.4) References of Indian periodical articles are quite less in percentage in the field of Molecular Biology

(C) In the case of instrumentation Engineering a large number of references are on patents

(C.1) Unlike Molecular Biology Researchers, Journal articles constitute to comparatively less in numbers

Now practically speaking, in case you are told to start a Library of a Newly build Molecular Biology Research Instiute, You will have a list of favourite journals. and you will not collect old journals beyond 7 years.

 It is like that.

I hope I could explain  it to you.

Siddhartha S. Ray, Calcutta <sidsray@gmail.com>