Probability theory provides a way to find and express our uncertainty in making decisions about a population from sample information. Probability is a number between 0 and 1. The highest value of any probability is 1.
Probability reflects the long-run relative frequency of the outcome.
The classical probability utilizes rules and laws. It involves an experiment. Relative probability is based on cumulated historical data. For example, your company wants to decide on the probability that its inspectors are going to reject the next batch of raw materials from a supplier. Data collected from your company record books show that the supplier had sent your company 80 batches in the past, and inspectors had rejected 15 of them.