Question on Functioning of a Search Engine

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hy newsgroup,

i've been asked a question and i cant get the essence of it. Can you
help me on it?

A student is very interested in search engines. But he finds that some
results are unusable. He decides to invent some parameters to compare
their performances.
First of all, he supooses that a document is "suitable" if it contains
informations he finds usable. If not, its "unsuitable". Afterwards, he
notices that every response to one of his queries is composed of liks to
"suitable" pages (he calles p the number of those links) and he gets
"unsuitable" results (he calls the sum of those pages n). Finally he
realizes that the response he gets to his query doesnt _always_ contain
the links to the total number of suitable documents of pages caches in
the database of the search engine. He calls "t" the number of total
"suitable" entries in the databbase.
Finally he conclues two criteria for the comparision of the engines:
* the "completude" x=p/t
* the "concentration" y= p/(p+n)
Finally he notices that in fact all he imagined is already existant.
Which are the english names for those two criterias?

thanks a lot in advance
so long

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