Let be the value of the variable of interest at a certain location . This value is unknown (e.g. temperature, rainfall, piezometric level, geological facies, etc.). Although there exists a value at location that could be measured, geostatistics considers this value as random since it was not measured, or has not been measured yet. However, the randomness of is not complete, but defined by a cumulative distribution function (CDF) that depends on certain information that is known about the value :