A prediction is a statement about a future event or data, often based upon experience or knowledge. It is a guess or forecast about what someone thinks will happen in the future, sometimes based on facts or evidence, but not always. Predictions can be useful to assist in making plans about possible developments. In a non-statistical sense, the term "prediction" is often used to refer to an informed guess or opinion. In statistics, prediction is a part of statistical inference, and it can be undertaken within any of the several approaches to statistical inference. In science, a prediction is a rigorous, often quantitative, statement, forecasting what would be observed under specific conditions. For example, according to theories of gravity, if an apple fell from a tree, it would be seen to move towards the center of the Earth with a specified and constant acceleration.