# A computer intrusion detection system (IDS) is designed to provide an alarm whenever there is unauthorized access into a computer system. A probabilistic evaluation of a system with two independently operating intrusion detection systems (a double IDS) was published. Consider a double IDS with system A and B. If there is an intruder, A sounds an alarm with probability .9 and system B sounds an alarm with probability .95. If there is no intruder, the probability system A will sound an alarm is .2 and system B is .1a). if there is an intruder, what is the probability that both systems sound an alarm?b) given that there is an intruder, what is the probability that at least one of the systems sounds an alarm?

Addition Rule of Probability

It simply refers to the likelihood of an event taking place whenever the occurrence of an event is uncertain. The probability of a single event can be calculated by dividing the number of successful trials of that event by the total number of trials.

Expected Value

When a large number of trials are performed for any random variable ‘X’, the predicted result is most likely the mean of all the outcomes for the random variable and it is known as expected value also known as expectation. The expected value, also known as the expectation, is denoted by: E(X).

Probability Distributions

Understanding probability is necessary to know the probability distributions. In statistics, probability is how the uncertainty of an event is measured. This event can be anything. The most common examples include tossing a coin, rolling a die, or choosing a card. Each of these events has multiple possibilities. Every such possibility is measured with the help of probability. To be more precise, the probability is used for calculating the occurrence of events that may or may not happen. Probability does not give sure results. Unless the probability of any event is 1, the different outcomes may or may not happen in real life, regardless of how less or how more their probability is.

Basic Probability

The simple definition of probability it is a chance of the occurrence of an event. It is defined in numerical form and the probability value is between 0 to 1. The probability value 0 indicates that there is no chance of that event occurring and the probability value 1 indicates that the event will occur. Sum of the probability value must be 1. The probability value is never a negative number. If it happens, then recheck the calculation.

A computer intrusion detection system (IDS) is designed to provide an alarm whenever there is unauthorized access into a computer system. A probabilistic evaluation of a system with two independently operating intrusion detection systems (a double IDS) was published. Consider a double IDS with system A and B. If there is an intruder, A sounds an alarm with

a). if there is an intruder, what is the probability that both systems sound an alarm?

b) given that there is an intruder, what is the probability that at least one of the systems sounds an alarm?

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