Density trace (boxcar weight function)                          [STB-16: snp6]
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         ^boxdetra^ varname width dtrace

Description
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^boxdetra^ calculates the density trace of "varname" using the boxcar weight 
function described in Chambers et. al. (1983).  "width" is a constant that
specifies the width of the density window around each point.  "dtrace" is 
a new variable that contains the density trace on output.  The data set is 
assumed to be sorted in the order of "varname".

This procedure can be regarded as a descriptive smoother of histograms.
Indeed this simple density estimator is like a histogram using bins of the
specified width but no fixed origin; the bin's windows run along the 
series with each observation at its midpoint allowing overlap.
The "smoothness" of the result can be regulated by changing the width of
the interval. Wide intervals produce smooth results; narrow intervals
result in noiser density values.

WARNING: This procedure performs conditional summaries for every observation
in the data set. Thus, the time it requires is proportional to the quantity 
of data. Please be patient.

Example
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  . ^boxdetra ozone 75 dtrace^

After some time (depending on the number of observations) the results are
placed in the new variable ^dtrace^. 

  . ^list ozone dtrace^

lists the values of the data variable and the density trace estimation 
results.

  . ^graph dtrace ozone^, xlab ylab s(.) c(J)

displays a graph of the density trace.


References
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   Chambers, J.M., W.S. Cleveland, B. Kleiner and P.A. Tukey (1983)
        Graphical Methods for Data Analysis. Wadsworth & Brooks/Cole
        Chap. 2: 9-46.

   Fox, J. (1990) Describing univariate distributions. In (Fox, J. &
        J.S. Long, Eds.) "Modern Methods of Data Analysis". Sage
        Chap. 2: 58-125.


Authors
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Isaias H. Salgado-Ugarte, Makoto Shimizu and Toru Taniuchi,
University of Tokyo, Faculty of Agriculture,
Dept. of Fisheries (Fax 81-3-3812-0529)


Also see
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    STB: snp6 (STB-16)
On-line: ^help^ for ^boxdetr2^, ^cosdetra^, ^kernsim^, ^kernepa^, ^kerngaus^, 
         ^adgakern^, ^adgaker2^