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# RE: st: RE: inflection point of sigmoid curves

 From "Lachenbruch, Peter" To "'statalist@hsphsun2.harvard.edu'" Subject RE: st: RE: inflection point of sigmoid curves Date Tue, 15 Jun 2010 08:53:20 -0700

```If you wish the inflection point of a normal curve you can take the second derivative of a normal density and find the inflection points are at mu+sigma and mu-sigma.

Tony

Peter A. Lachenbruch
Department of Public Health
Oregon State University
Corvallis, OR 97330
Phone: 541-737-3832
FAX: 541-737-4001

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Austin Nichols
Sent: Monday, June 14, 2010 2:17 PM
To: statalist@hsphsun2.harvard.edu
Subject: Re: st: RE: inflection point of sigmoid curves

Johannes Schmid--
Try these silly examples:

drawnorm x, mean(10) n(1000) clear
sort x
g y=(_n+1)/(_N+2)
reg x c.y##c.y##c.y
loc ybar=-2*_b[c.y#c.y]/(6*_b[c.y#c.y#c.y])
set obs `=_N+1'
replace y=`ybar' in l
ipolate x y, gen(px)
loc xbar=px[_N]
lpoly y x, nosc xli(`xbar')

webuse nhanes2, clear
ren  bpsystol y
ren age x
reg x c.y##c.y##c.y
loc ybar=-2*_b[c.y#c.y]/(6*_b[c.y#c.y#c.y])
set obs `=_N+1'
replace y=`ybar' in l
ipolate x y, gen(px)
loc xbar=px[_N]
lpoly y x, nosc xli(`xbar')

On Mon, Jun 14, 2010 at 3:12 PM, Nick Cox <n.j.cox@durham.ac.uk> wrote:
> I am not aware of a unique recipe for sigmoid curves. But in general fit
> a suitable function, apply calculus and plug in suitable parameter
> values.
>
> Nick
> n.j.cox@durham.ac.uk
>
> Johannes Schmid
>
> Is it possible to calculate the inflection point in sigmoid curves in
> STATA,
> and how can it be done?
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```