How much do you understand the subject _____? (Fill in the blank for your own example.)
If you were looking up something about that subject, you would want something at your level. If you are already very knowledgeable in the subject, you would want something more detailed than if you are just learning about it. All too often though, reference materials go way too deep for the beginner.
A example of this problem is Wikipedia. It can be quite useful for looking some things up, but beware. It is often not very useful for a beginner in a technical subject. Why? Because the write-up then often is very technical. If you want a good introduction to it, you may well be out of luck.
For example, here is the first paragraph of the article phylum (http://en.wikipedia.org/wiki/Phylum): ‘In biology, a phylum (/ˈfaɪləm/; plural: phyla)[note 1] is a taxonomic rank below kingdom and above class. Traditionally, in botany the term division is used instead of “phylum”, although in 1993 the International Botanical Congress accepted the designation “phylum”. The kingdom Animalia contains approximately 35 phyla; the kingdom Plantae contains 12 phyla. Current research in phylogenetics is uncovering the relationships between phyla, which are contained in larger clades, like Ecdysozoa and Embryophyta.’
If you are not particularly knowledgeable about biology, how many words would you have to look up in order to completely understand this paragraph?
Wikipedia would serve better if it had non-technical versions of pages or if technical pages started with a non-technical definition.