# Return period

A **return period**, also known as a **recurrence interval** or **repeat interval**, is an average time or an estimated average time between events such as earthquakes, floods,[1] landslides,[2] or river discharge flows to occur.

It is a statistical measurement typically based on historic data over an extended period, and is used usually for risk analysis. Examples include deciding whether a project should be allowed to go forward in a zone of a certain risk or designing structures to withstand events with a certain return period. The following analysis assumes that the probability of the event occurring does not vary over time and is independent of past events.