http://www.infotoday.com/searcher/jan02/block.htm
I often suggested to students that information is a lot like pizza — the hungrier you are, the more you eat. The more thorough your search needs to be, the more you need to search through all the available resources. This is my guess about what formats of information occupy what percentage of the total of information cumulated over the past 3 centuries. I believe documents produced by local, national and international governments over the centuries are the largest single source of information, followed by books and periodicals. Even with well over a billion pages, and adding sites at the rate of millions a day, the Internet still has a lot of catching up to do before it can compete. The remaining small segments include things like dissertations, conference papers, videos, movies, photographs, maps, databases, etc.
Each individual piece of the information pizza can be sliced even thinner. Even a tiny slice like magazines or journals can be subdivided into all the different databases that index or abstract them — Medline, ERIC, Biological Abstracts, Agricola. If you really want a thorough search, you need to check every likely itty-bitty slice.
These are the original rules of information, pretty much as I scrawled them and copied them off. They've grown a bit since then.
1. Go where it is.
2. The answer you get depends on the question you ask.
3. Research is a multi-stage process.
4. Ask a librarian