Arun wrote:
I am dealing with huge datasets for which I have to
estimate percentile exposure time within two to three
subsets. I noticed something that happened there and
tried duplicating it with a stata example dataset
[webuse beetle] and I noticed it again. Attached below
is the do file modified for beetle dataset.
/* Stata/IC 10.0 for Macintosh */
/* Do file to assess LD10-90 for beetle.dta */
/* Obtain beetle from webuse beetle */
use ~/desktop/beetle.dta
gen p=r/n
forvalues i=1/3 {
glm p ldose if beetle==`i', f(b)
forvalues j=0.1(0.1)0.9 {
local k=`j'*100
display as text "LD `k' = " as result (log(`j'/(1-`j'))-_b[_cons])/_b [ldose]
}
}
A part of the result for beetle.dta looks like this...
...
LD 10 = 1.7169257
LD 20 = 1.7389921
LD 30 = 1.7536588
LD 40 = 1.7656816
LD 50 = 1.7767148
LD 60 = 1.787748
LD 70 = 1.7997708
LD 80 = 1.8144375
LD 89.99999999999999 = 1.8365039
Why is this last LD 90 showing as 89.99... ... .
Am I doing something odd (bad math) in the do file?
--------------------------------------------------------------
You do things right, but computers (not only Stata)
do have limited precision; see, e.g.,
http://www.stata.com/support/faqs/data/prec.html <http://www.stata.com/support/faqs/data/prec.html> .
Take a look at this:
. local k=1/3
. display `k'
.33333333
. display "`k'"
.3333333333333333
If a macro is expanded to a text string it contains the
result in double precision. If you let -display- display
it, you see fewer digits. This means that you could write
display "LD " `k' " = ..."
rather than
display "LD `k' = ..."
to get:
LD 90 = 1.8365039
Hope this helps
Svend
________________________________________________________
Svend Juul
Institut for Folkesundhed, Afdeling for Epidemiologi
(Institute of Public Health, Department of Epidemiology)
Vennelyst Boulevard 6
DK-8000 Aarhus C, Denmark
Phone, work: +45 8942 6090
Phone, home: +45 8693 7796
Fax: +45 8613 1580
E-mail: [email protected]
_________________________________________________________
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