[R] confidence interval for logistic joinpoint regression from package ljr

[Vito Muggeo (UniPa)]

dear M.,
I do not know how to get the SE for the joinpoint (or breakpoint) from 
your ljr fit. However you can find useful the segmented package which 
works for any GLM (including the logistic one) and it returns 
(approximate) StErr (and Conf Int) also for the joinpoint (breakpoint in 
the segmented package). For some examples, see the R news paper
http://cran.r-project.org/doc/Rnews/Rnews_2008-1.pdf

Also, as regards to the APC, you could find interesting the following note

http://onlinelibrary.wiley.com/doi/10.1002/sim.3850/abstract

hope this helps you,
vito


Il 30/11/2010 23.54, moleps ha scritto:
> I´m trying to run a logistic joinpoint regression utilising the ljr package. I´ve been using the forward selection technique to get the number of knots for the analysis, but I´m uncertain as to my results and the interpretation. The documentation is rather brief ( in the package and the stats in medicine article is quite technical) and without any good examples. At the moment I´m thinking
> 1)find the number of knots both using forward and backward techniques and see if they are close
> 2)present the annual percent change (APC) for each of the intervals, ie my present data (1950-2010 in 5 year intervals) is giving me
>
>         Variables          Coef
> b0     Intercept -131.20404630
> g0             t    0.06146463
> g1 max(t-tau1,0)   -0.51582466
> g2 max(t-tau2,0)    0.43429615
>
> Joinpoints:
>
> 1 tau1= 1990.5
> 2 tau2= 1995.5
>
> APC 1950->1990=exp(0.06)=1.06-->6%
> 1990-1995=exp(0.06-0.51)=exp(-0.45)=0.63-->  -37%
> 1995-2010=exp(0.06-0.51+0.43)-->-2%
>
> 3) Preferably a confidence interval for the APC should be given. However, this I havent figured out yet.
>
> //M
>
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-- 
====================================
Vito M.R. Muggeo
Dip.to Sc Statist e Matem `Vianelli'
Università di Palermo
viale delle Scienze, edificio 13
90128 Palermo - ITALY
tel: 091 23895240
fax: 091 485726/485612
http://dssm.unipa.it/vmuggeo