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RE: st: simulated data for logistic regression... remedial algebra help?


From   "Daniel Waxman" <dan@amplecat.com>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: simulated data for logistic regression... remedial algebra help?
Date   Mon, 19 Nov 2007 22:37:22 -0500

*Success*  

Steven,
 
Thank you very much for the help.
Quadratic formula ... brings back memories.

There was a wicked exponentiation problem that took a few hours to figure
out (beware of how Stata evaluates the exponentiation of a negative
number... e.g. -2^2 evaluates to -4)

Anyway, here is the result:

****

clear
set obs 1000
local odds=2
local proportion_positive= .10
local mortality = .05

gen test=uniform()<`proportion_positive'

local k1=`mortality'/(1-`proportion_positive')
local k2=(`proportion_positive'/(1-`proportion_positive'))
di `k1'
di `k2'

local a=`k2'
local b=(-2)*`k2'-`k1'-1
local c=2*`k1'

gen probability_positive = ((-`b') - (sqrt((`b')^2-(4*`a'*`c'))))/(2*`a')
gen probability_negative = (`k1')-((`k2')*probability_positive)
gen died=uniform() < cond(test==0,probability_negative,probability_positive)

logistic died test

******************

Thanks much!

Dan


-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Steven Joel
Hirsch Samuels
Sent: Monday, November 19, 2007 12:07 PM
To: statalist@hsphsun2.harvard.edu
Subject: Re: st: RE: simulated data for logistic regression... remedial
algebra help? 

>
>> On Nov 19, 2007, at 9:49 AM, Steven Joel Hirsch Samuels wrote:
>>>

>  p1 = (p -(1 -w)p2)/w  =B    (you left out the 1-w term in your k2  
> coefficient)
>>>

No you didn't, Daniel.  Sorry.

-Steven
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