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From |
n j cox <n.j.cox@durham.ac.uk> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: loops |

Date |
Mon, 24 Sep 2007 19:34:44 +0100 |

I agree with Svend: this is all a matter of precision.

Of the nine numbers 0.1(0.1)0.9, only one (0.5) has an

exact binary equivalent.

Partly because of this, I typically loop over integers:

use ~/desktop/beetle.dta

gen p = r/n

forvalues i = 1/3 {

glm p ldose if beetle==`i', f(b)

forvalues j = 1/9 {

display as text "LD `j'0 = " ///

as res logit(`j'/10) - _b[_cons])/_b[ldose]

}

}

Note in passing that there is a -logit()- function.

Nick

n.j.cox@durham.ac.uk

Svend Juul replied to Arun:

--------------------------------------------------------------------------------

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

*

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**Follow-Ups**:**Re: st: loops***From:*Arun Rajamohan <arajamoh@uwo.ca>

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