Aim: Real data often contain errors of various sorts. These may be due to instrumental failure, contamination, mis-typing, etc. Some data have values which are significantly different to that of the bulk of the data. These extreme values are referred to as outliers, and may affect various statistics severely (i.e. the standard deviation).
Extreme climatic events may have severe impacts on the society, economy, and the ecosystems, and are often the focus of risk management. For instance, hydroelectric dams may sustain water up to a critical level: how often can one expect high rainfall to cause the water level to exceed this threshold?