Posted by
Hector Maletta on
Aug 23, 2006; 4:10pm
URL: http://spssx-discussion.165.s1.nabble.com/Re-interpretation-of-the-warning-message-tp1070503p1070506.html
I am puzzled too. That makes two of us, Lana. Just to speculate about it I
may offer this (speculating is free): the % of the event is about 9%, and
even less than that would have reunification as a reason for discharge, so
perhaps having few cases involved is not really to discard out of hand,
especially if the relationship between the two is not strong. Try a 2x2
table of reunification as a reason (Yes/No) and reentry in foster care
within 12 months (Yes/No) and check the degree of association and the number
of Yes-Yes cases. If the association is weak, and there are relatively few
Yes-Yes cases, we may be into something. I don't bet much on it, but it is a
possibility.
Hector
-----Mensaje original-----
De: Yampolskaya, Svetlana [mailto:
[hidden email]]
Enviado el: Wednesday, August 23, 2006 11:45 AM
Para: Hector Maletta
Asunto: RE: interpretation of the warning message
Hector,
Thank you for your reply but I am still puzzled because I have 34,830
cases of children who exited foster care during two years. 3,022
reentered foster care within 12 months. So I am trying to look at time
to reentry. In this model I used only one covariate - reunification as a
reason for discharge. I coded that variable 1 - reunification as a
reason for discharge and 0 - other reasons. When I do the same analysis
and use a different covariate - placement with relatives as a reason for
discharge - the model runs without any problem. Multivariate model runs
without problems too.
Any advise will be greatly appreciated,
Lana
-----Original Message-----
From: Hector Maletta [mailto:
[hidden email]]
Sent: Wednesday, August 23, 2006 10:32 AM
To: Yampolskaya, Svetlana;
[hidden email]
Subject: RE: interpretation of the warning message
You probably were doing it stepwise, and the procedure failed to
converge at some step, so it judged it wise to stop at that step and not
trying to introduce further variables. Or perhaps you were doing it in a
single step, and the model simply did not converge. It just happens
sometimes, when the data fail to fit the model.
The reasons may vary. Increasing the number of iterations seldom helps.
Relaxing the convergence criterion is close to cheating and seldom
advisable. Also, it seldom works either (the failure to converge is
often by a much wider margin than any reasonable relaxation you may
introduce in the convergence criterion). Perhaps you have too few cases
to arrive at a significant solution, or perhaps the model is poorly
specified. Perhaps the hazards are not proportional, so Cox does not
apply unless some time-related covariates are introduced. Try to modify
the model, e.g. introducing time-varying covariates, or changing the
list of covariates by withdrawing some of them that seem to have less
strong relationship with the event of interest. Perhaps you may try some
simpler models first, to see whether the covariates fit the data
(predict survival) one by one, or in pairs, before running a more
complicated model. Sorry to say I do not have a magic bullet, but this,
as poetry, is more perspiration than inspiration.
Hector
-----Mensaje original-----
De: SPSSX(r) Discussion [mailto:
[hidden email]] En nombre de
Yampolskaya, Svetlana Enviado el: Wednesday, August 23, 2006 11:20 AM
Para:
[hidden email]
Asunto: Re: interpretation of the warning message
Dear List,
I am trying to do Cox regression (bivariate analysis) and I have this
message that I cannot interpret.
Can you please help and tell me what was wrong?
Warnings
Since coefficients did not converge, no further models will be fitted.
Thank you,
Lana