In some embodiments, an online fraud prevention system combines the output of several distinct fraud filters, to produce an aggregate score indicative of the likelihood that a surveyed target document (e.g. webpage, email) is fraudulent. Newly implemented fraud filters can be incorporated and ageing fraud filters can be phased out without the need to recalculate individual scores or to renormalize the aggregate fraud score. Every time the output of an individual filter is calculated, the aggregate score is updated in a manner which ensures the aggregate score remains within predetermined bounds defined by a minimum allowable score and a maximum allowable score (e.g., 0 to 100).