Occurrence of a particular event can change gradually either stronger or weaker over time.The occurrence of an event can also depend on other factors. For instance, if an event takes place in the presence of a strong secondary factor, such as weather or the time of day, it may be more likely to happen than if it was in isolation. Similarly, the presence of necessary resources or materials may increase the probability that an event will occur.
Finally, the frequency of occurrence of an event may also be affected by the intentional or unintentional actions of people. People may take measures to increase the chance of a particular event from taking place, and likewise may be able to decrease the chance of an event occurring. This could include providing resources or materials, changing the environment, or even taking direct action.