Several agricultural field experiments have run for more than 100 years, but much shorter experiments may qualify as "long-term" in other disciplines. An experiment is "a set of actions and observations", implying that one or more treatments (fertilizer, subsidized school lunches, etc. ) is imposed on the system under study. Long-term experiments therefore contrast with nonexperimental long-term studies in which manipulation of the system studied is impossible or undesirable (field observations of chimpanzee behavior).