Double mass plot
doubmass yvar xvarlist [if exp] [in range] [, ratio scatter_options ]
doubmass plots the cumulative sum of yvar against the average of the cumulated sums of each variable in xvarlist. If xvarlist contains just one variable xvar, doubmass reduces to a plot of the cumulative sum of yvar against the cumulative sum of xvar.
It is assumed that data are tsset and that no more than one panel is considered in any call. Otherwise put, doubmass expects a data structure in which contemporaneous panels constitute parallel time series.
In hydrology double mass plots are a standard method of checking the consistency of time series. For precipitation or stream discharge in particular, nearby sites often have averages differing by a multiplier, so that their ratio is constant, to a good approximation. Any breaks of slope on the graph may indicate discontinuities such as station shift, instrument breakdown, or changes in site conditions.
This technique appears not to be covered in literature on cusum charts for quality control (e.g. Woodward and Goldsmith 1964).
ratio plots the slope
cumulative sum of yvar -------------------------------------- average of cumulative sums of xvarlist
against average of cumulative sums of xvarlist.
graph_options are any of the options allowed with scatter.
. doubmass site1 site2
Nicholas J. Cox, Durham University, U.K. firstname.lastname@example.org
Clarke, R.T. 1994. Statistical Modelling in Hydrology. Chichester: John Wiley,
Dingman, S.L. 1994. Physical Hydrology. Englewood Cliffs, NJ: Prentice-Hall.
Dunne, T. and L.B. Leopold. 1978. Water in Environmental Planning. San Francisco: W.H. Freeman.
McCuen, R.H. 1998. Hydrologic Analysis and Design. Upper Saddle River, NJ: Prentice-Hall. pp.193-197.
Searcy, J.K. and C.H. Hardison. 1960. Double-mass curves. United States Geological Survey Water-Supply Paper 1541B.
Woodward, R.H. and P.L. Goldsmith. 1964. Cumulative Sum Techniques. Edinburgh: Oliver and Boyd for Imperial Chemical Industries.
help for scatter