Double mass analysis
Double mass analysis  is a commonly used data analysis approach for investigating the behaviour of records made of hydrological or meteorological data at a number of locations. It is used to determine whether there is a need for corrections to the data - to account for changes in data collection procedures or other local conditions. Such changes may result from a variety of things including changes in instrumentation, changes in observation procedures, or changes in gauge location or surrounding conditions. Double mass analysis for checking consistency of a hydrological or meteorological record is considered to be an essential tool before taking it for analysis purpose. This method is based on the hypothesis that each item of the recorded data of a population is consistent.
An example of a double mass analysis is a "double mass plot", or "double mass curve". For this, points and/or a joining line are plotted where the x- and y- coordinates are determined by the running totals of the values observed at two stations. If both stations are affected to the same extent by the same trends then a double mass curve should follow a straight line. A break in the slope of the curve would indicate that conditions have changed at one location but not at another. This technique is based on the principle that when each recorded data comes from the same parent population, they are consistent.
- Searcy, J.K. and C.H. Hardison (1960). Double-mass curves. U.S. Geological Survey Water-Supply Paper 1541-B'
- Wilson, E.M. (1983) Engineering Hydrology, 3rd edition. Macmillan Press, London. p.27
- Dubreuil P. (1974) Initiation à l'analyse hydrologique Masson& Cie et ORSTOM, Paris.
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