Some events are more likely to happen than others, while some events are equally likely.
It is possible to measure the likelihood of specific events. Sometimes an experiment produces data that do not exactly match a theoretical probability. When there is no direct observation that will answer a particular question, it is often possible to gather data which can be used to answer the question. Working with the data often generates additional questions. You can gather data about every member of a group, or you can use a representative sample from that group.