There are two ways in which the probability of an event can be calculated. One way is to use data obtained from a chance experiment to predict the probability of the event in the future. The other approach is to systematically list all of the possible outcomes of the chance experiment in order to work out the probability that the desired event will occur. Experimental probability describes what actually happens when a chance experiment is conducted. Sample spaces can take the form of a list, table, grid or tree diagram. It is important to be systematic when preparing a sample space, since all of the possible outcomes must be identified if the theoretical probability is to be calculated accurately. Tree diagrams provide a useful way of representing multi-stage probability experiments. The first two branches of the tree show the possible outcomes of the first stage of the experiment—the first card selected is either red or black.