{smcl}
{* 9 Feb 2009}{...}
{hline}
{cmd:help doubmass}
{hline}
{title:Double mass plot}
{p 8 16 2}
{cmd:doubmass}
{it:yvar}
{it:xvarlist}
[{cmd:if} {it:exp}]
[{cmd:in} {it:range}]
[{cmd:,}
{cmd:ratio}
{it:scatter_options} ]
{title:Description}
{p 4 4 2}
{cmd:doubmass} plots the cumulative sum of {it:yvar} against the average
of the cumulated sums of each variable in {it:xvarlist}. If
{it:xvarlist} contains just one variable {it:xvar}, {cmd:doubmass}
reduces to a plot of the cumulative sum of {it:yvar} against the
cumulative sum of {it:xvar}.
{p 4 4 2}
It is assumed that data are {help tsset} and that no more than one panel
is considered in any call. Otherwise put, {cmd:doubmass} expects a data
structure in which contemporaneous panels constitute parallel time
series.
{title:Remarks}
{p 4 4 2}
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.
{p 4 4 2}
This technique appears not to be covered in literature on cusum charts for
quality control (e.g. Woodward and Goldsmith 1964).
{title:Options}
{p 4 8 2}
{cmd:ratio} plots the slope
cumulative sum of {it:yvar}
{hline 38}
average of cumulative sums of {it:xvarlist}
{p 4 8 2}
against average of cumulative sums of {it:xvarlist}.
{p 4 8 2}
{it:graph_options} are any of the options allowed with {help scatter}.
{title:Example}
{p 4 8 2}{cmd:. doubmass site1 site2}
{title:Author}
{p 4 4 2}Nicholas J. Cox, Durham University, U.K.{break}
n.j.cox@durham.ac.uk
{title:References}
{p 4 8 2}
Clarke, R.T. 1994.
{it:Statistical Modelling in Hydrology.}
Chichester: John Wiley,
{p 4 8 2}
Dingman, S.L. 1994.
{it:Physical Hydrology.}
Englewood Cliffs, NJ: Prentice-Hall.
{p 4 8 2}
Dunne, T. and L.B. Leopold. 1978.
{it:Water in Environmental Planning.}
San Francisco: W.H. Freeman.
{p 4 8 2}
McCuen, R.H. 1998.
{it:Hydrologic Analysis and Design.}
Upper Saddle River, NJ: Prentice-Hall.
pp.193-197.
{p 4 8 2}
Searcy, J.K. and C.H. Hardison. 1960.
Double-mass curves.
{it:United States Geological Survey Water-Supply Paper} 1541B.
{p 4 8 2}
Woodward, R.H. and P.L. Goldsmith. 1964.
{it:Cumulative Sum Techniques.}
Edinburgh: Oliver and Boyd for Imperial Chemical Industries.
{title:Also see}
{p 4 13 2}help for {help scatter}