Dear Keith
My post would do what you want, but normal spline regression is far far far more stable. So Nick was right (as usual) in pointing you to -mkspline-. I would only use -nl- if the location of the knot were my primary interest. I would still prefer to use somewhat plausible locations (like equally spaced knots) rather than -nl-, even if I did not know the exact location, if the exact locations of the knots are not of substantial interest (as is usually the case). If you just want to fit a flexible line through your data you could have a look at -locpoly-.
Hope this helps,
Maarten
-----Original Message-----
From: [email protected] [mailto:[email protected]]On Behalf Of keith Farnsworth
Sent: dinsdag 24 mei 2005 14:13
To: [email protected]
Subject: Re: st: RE: broken stick (piecewise linear) regression
Thanks, Nick, but I looked at mkspline before and realised that it
specifically requires the knot point, whereas the problem I have,
addressed by Bacon and Watts, is to find the best fit knot point i.e -
estimating the transition between two intersecting straight lines.
Unless there is something you can do with mkspline that is not in the
documentation, I don't see that it can fit the knot.
Any other idea?
Keith.
On Tuesday, May 24, 2005, at 12:55 Europe/Dublin, Nick Cox wrote:
> . search mkspline
>
> should help. See both entries.
>
> Nick
> [email protected]
>
> keith Farnsworth
>
>> I intend to fit a broken stick model following Bacon and Watts
>> "Estimating the transition between two intersecting straight lines",
>> Biometrika 1971. I have seen code for this to use the S or R
>> package's
>> nls, which I suspect uses maximum likelihood (but not sure) [see
>> http://www.biostat.wustl.edu/archives/html/s-news/2000-04/
>> msg00215.html].
>>
>> I am not an S user, I am a Stata enthusiast!
>> However, I found no reference to this kind of model in Stata
>> searches.
>> I hope someone with more experience than me can explain how
>> Stata will
>> implement the Bacon and Watts model.
>
> *
> * For searches and help try:
> * http://www.stata.com/support/faqs/res/findit.html
> * http://www.stata.com/support/statalist/faq
> * http://www.ats.ucla.edu/stat/stata/
>
>
Dr Keith Farnsworth
Lecturer in Mathematical and Behavioural Ecology
Queens University Belfast
Medical Biology Centre
97 Lisburn Road
Belfast BT9 7BL
Northern Ireland
phone +44 (0)2890 272352
fax +44 (0)2890 236505
*
* For searches and help try:
* http://www.stata.com/support/faqs/res/findit.html
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/
*
* For searches and help try:
* http://www.stata.com/support/faqs/res/findit.html
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/
