Classifiers are very handy general-purpose tools for computing "the degree of connectedness" in relations between observed variables (see this application in marketing).

An example where a classifier is handy to apply, is in data-mining of web-links.

Suppose in this small example that you have the following questions:

• How often is a product from our product line Referred to, among a collection of webpages?

• How often is the Referral due to a link to one of our published images of a product?

• How often is the Referral due to mentioning of a name of one of our brands?

Classifiers are probability models. So instead of yielding the ‘how often’, they compute the