In class we've learnt to use as to compare two things when expressing they have a similar proportion as in
My car is as expensive as yours.
In negative statements we can also use so...as as in
He is not so/as intelligent as his sister is
Click here for a review of the different uses of as and so...as.
Learning English. BBC World Service
http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv158.shtml
