**3. Probability**

"Probability" has become one of the key methods for statistics in dentistry. Often statistical analysis is paralyzed without theory of probability [2].

The probability of an event is its chance of occurrence, measured on a scale from 0 (never occurs) to 1 (always occurring). There are two views we may take of probability. One is that it is the long-term frequency of an event, e.g., in a long series of coin tosses, heads should occur about half of the time, so we write P (Head) = 0.5. The second and broader, view is that probability is a subjective measure of our belief in the chances of an event occurring, e.g., "I believe that there is a 30% chance of a practical AIDS vaccine by the year 2025". Here, because the event can only occur once, there is no sensible long-term relative frequency view, but such subjective probabilities are useful when making decisions, e.g. planning future medical facilities. Of course, such subjective probabilities are most likely to be accurate when based on good long-term relative frequency information [3]. Predictions are taking the form of probabilities. We use probabilities to predict the likelihood of an earthquake, rain, or whether you are going to get an A in exams. Dentists use percentages to assess the risk of appliance that may trigger gum disease to use alternates. The investment schemes use the probability to determine the rate of return on the investment of the client. You could use the chance to decide whether to purchase a profitable commercial land. In your analysis of statistics, you will use the power of mathematics through probability calculations to evaluate and interpret your results [4].
