Thanks to Kit Baum, the program -skewplot- has
been updated on SSC, so that it now uses Stata 8's
new graphics.
At the same time, the previous version of
-skewplot- has been renamed -skewplot6-
as a flag that it requires Stata 6 and
so may be used in either Stata 6 or Stata 7.
(I am aware of various other ways of keeping
different versions of a program equally
accessible, but this was most convenient
to me in this instance.)
-skewplot- has some affinity with -symplot-,
which has been in official Stata for some
time, so let us start from there. The idea behind
-symplot- is to plot
y = upper quantile - median
versus
x = median - lower quantile
where the upper and lower quantiles
are corresponding pairs such as
maximum and minimum
second largest and second smallest
etc.
A characteristic of a symmetric distribution
is evidently that y = x, so that this sloping
line is the natural reference line.
However, -symplot- as implemented in Stata
supports only the plotting of one set of
data, so that comparison requires production
of several graphs. In addition, the sloping
reference is, in my experience, more awkward
to work with than the horizontal reference of
a display of
(upper quantile + lower quantile) / 2
versus
upper quantile - lower quantile
In a symmetric distribution, the first variable
produces equal estimates of the centre of the
distribution.
-skewplot- supports the plotting of several
variables or of several subsets of a variable.
To install in Stata 8, type
. ssc inst skewplot
or
. ssc inst skewplot, replace
as appropriate.
Nick
[email protected]
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