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Double mass plot

        doubmass yvar xvarlist [if exp] [in range] [, ratio scatter_options ]


Description 

    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.


Remarks

    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).


Options 

    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.


Example

    . doubmass site1 site2


Author 

    Nicholas J. Cox, Durham University, U.K.
    n.j.cox@durham.ac.uk


References 

    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.


Also see 

    help for scatter