The Computer Security is a part of Computer Science that addresses all the problems that exists within the computer security domain. Now days, more than ever, the computer viruses, worms, hackers, crackers, electronic eavesdropping and electronic fraud, have become the new problems that the Network Computer Security Science must solve. The Intrusion Detection System has become a needful component in a well formed network security policy. Although, intrusion detection is not a new technology, since it has been research subject for almost 30 years now, it has been liable to the false positive problem i.e. base-rate fallacy.