(1) Determining the extent to which one randomly varying quantity can be expressed as a function of another or both can be expressed as functions of a third, non random quantity. This definition is extended to situations involving several variables by applying it to each different pair of variables in the set. Correlation is linear, quadratic, etc. , if the function is linear, quadratic, etc. The extent is measured by a quantity called the correlation constant or correlation coefficient. (2) The removal of discrepancies that may exist among survey data so that all data agree without apparent error. The terms coordination and correlation are usually applied to the harmonising of surveys of adjacent regions or of different surveys over the same region. Two or more such surveys are coordinated when they are computed on the same datum; they are correlated when they are adjusted together.