It's hard to answer your question without knowing at least the domain you're working at, so you may have a look at the following article, which provides overview of existing outlier detection techniques by classifying them along different dimensions:

VARUN CHANDOLA, ARINDAM BANERJEE, VIPIN KUMAR "Outlier Detection : A Survey" http://www.bradblock.com.s3-website-us-west-1.amazonaws.com/Outlier_Detection_A_Survey.pdf

(or just google this article)

You'll probably find what you need following the references.