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From |
Maarten buis <maartenbuis@yahoo.co.uk> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: spline |

Date |
Wed, 6 Feb 2008 14:39:51 +0000 (GMT) |

--- "A. Manzoni" <A.Manzoni@uvt.nl> wrote: > Here is my case: I want to model recall bias in retrospective > surveys, looking at the length of the recall period (i.e. distance > between the events reconstructed and the interview) and I was > suggested to try to use a spline model. > > I have some theories concerning the shape of the recall (i.e. for > example I may think that from 10 years onwards it doesn't make any > difference how much the time distance to the interview is), but I > know it should be somehow possible to get the number and the position > of knots which better describe what's going on, besides the shape of > the effect. With splines there is a number of issues: o the number of knots o the location of the knots o is the curve between knot linear or quadratic or cubic or... A simple representation of your theory would be a linear spline with a single knot at 10 years. You would make these variables like this: mkspline yr1 10 yr2 = year If your theory is true than you would expect that the coefficient of yr2 is 0 An elaboration would be that you want to estimate where that knot is. This was discussed in this post: http://www.stata.com/statalist/archive/2008-01/msg01006.html It may be that you just want to allow the effect of time to have some flexible non-linear effect. In that case you could have a look at Patrick Royston's -mvrs- command (see: -findit mvrs-), which is described in (Royston and Sauerbrei 2007). Hope this helps, Maarten Patrick Royston and Willi Sauerbrei (2007) "Multivariable modeling with cubic regression splines: A principled approach". The Stata Journal, 7(1): 45-70. ----------------------------------------- Maarten L. Buis Department of Social Research Methodology Vrije Universiteit Amsterdam Boelelaan 1081 1081 HV Amsterdam The Netherlands visiting address: Buitenveldertselaan 3 (Metropolitan), room Z434 +31 20 5986715 http://home.fsw.vu.nl/m.buis/ ----------------------------------------- __________________________________________________________ Sent from Yahoo! Mail - a smarter inbox http://uk.mail.yahoo.com * * 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/

**References**:**st: spline***From:*"A. Manzoni" <A.Manzoni@uvt.nl>

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