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. For example, medical doctors sometimes assign subjective probabilities to the length of life expectancy for people having cancer.
Weather forecasting is another example of subjective probability.
Experiment is an activity that is either observed or measured, such as tossing a coin, or drawing a card. An event is a possible outcome of an experiment. For example, if the experiment is to sample six lamps coming off a production line, an event could be to get one defective and five good ones. For example, suppose that the experiment is to roll a die. A sample space is a complete set of all events of an experiment.