1. A method of evaluating fraud risk of an electronic commerce transaction, the method comprising the computer-implemented steps of: receiving transaction data that defines the electronic commerce transaction; determining a first fraud risk score value associated with the electronic commerce transaction based on applying a plurality of tests to the transaction data, wherein each of the plurality of tests determines whether the transaction data appears to represent a genuine transaction based on specified criteria; determining a second fraud risk score value associated with the electronic commerce transaction based on a comparison of the transaction data to historical transaction data; combining the first fraud risk score value and the second fraud risk score value using a statistical model to result in creating a model score value; and blending the model score value with one or more merchant-specific threshold values to result in creating and storing a final fraud risk score value for the electronic commerce transaction; wherein the step of blending the model score value comprises the steps of: determining in which fraud risk zone, of two or more fraud risk zones, the boundaries of which are determined by the one or more merchant-specific threshold values, the model score value belongs; and applying a policy corresponding to the determined fraud risk zone, wherein the policy dictates a magnitude and an allowable direction of influence applied by a heuristic model and a statistical model.