The behavior and nature of attacks and threats to computer network systems have been evolving rapidly with the advances in computer security technology. At the same time however, computer criminals and other malicious elements find ways and methods to thwart such protective measures and find techniques of penetrating such secure systems. Therefore adaptability, or the ability to learn and react to a consistently changing threat environment, is a key requirement for modern intrusion detection systems. In this paper we try to develop a novel metric to assess the performance of such intrusion detection systems under the influence of attacks. We propose a new metric called feedback reliability ratio for an intrusion detection system. We further try to modify and use the already available statistical Canberra distance metric and apply it to intrusion detection to quantify the dissimilarity between malicious elements and normal nodes in a network.