I think I have discovered a rarely-encountered bug/feature in the Stata
-graph- command. (I use the very latest Stata 7 under Windows 98 SE.) It
appears that, if there is only one observation in the selected sample, and
-ylog- (or -xlog-) is specified, and the single y-value (or x-value) is in
the open interval between one and zero, then -graph- complains of
non-positive values. For instance, in the -auto- data, I might define a
variable -gpm- by typing:
. drop gpm
. gene gpm=1/mpg
. summ gpm
Variable | Obs Mean Std. Dev. Min Max
-------------+-----------------------------------------------------
gpm | 74 .0501928 .0127986 .0243902 .0833333
As you can see, -gpm- (gallons per mile) has values all in the open
interval from 0 to 1. If I then use this variable in a -graph- command with
one observation and the option -xlog- or -ylog-, then the result is as follows:
. grap weight gpm in 1,xlog
nonpositive values encountered
r(411);
. grap gpm weight in 1,ylog
nonpositive values encountered
r(411);
I get similar results if I restrict the -graph- to any other observation.
However, -graph- doesn't seem to mind if I type
. grap gpm weight in 1,xlog
. grap weight gpm in 1,ylog
in which case graphs are drawn. Nor is there any problem if more than one
observation is included, as follows:
. grap weight gpm in 1/2,xlog
. grap gpm weight in 1/2,ylog
It therefore seems that the problem -graph- has is specifically with the
non-positive logs in single-observation sample sets.
The bug/feature I have found is, of course, rarely encountered in day to
day statistical work, because very few people draw graphs with one
observation. (I discovered it today in the course of writing a cert script
for my -smileplot- package, downloadable from SSC, which plots P-values on
a reverse log scale against the corresponding estimates.) However, in the
interests of completeness, it would be ideal if this bug/feature was fixed,
before it confuses too many more unwary cert script writers.
Best wishes
Roger
--
Roger Newson
Lecturer in Medical Statistics
Department of Public Health Sciences
King's College London
5th Floor, Capital House
42 Weston Street
London SE1 3QD
United Kingdom
Tel: 020 7848 6648 International +44 20 7848 6648
Fax: 020 7848 6620 International +44 20 7848 6620
or 020 7848 6605 International +44 20 7848 6605
Email: [email protected]
Opinions expressed are those of the author, not the institution.
