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 t0.06146463
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
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http://dssm.unipa.it/vmuggeo
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