Dear statalist,
Using stata 7, I am trying to perform a logistic regression analysis on a
dataset, covering several years, consisting of physician visit data and
various demographic or visit characteristic
variables. My basic unit (observation) is an individual patient visit to a
physician. Each visit consists of a different patient (no repeated visits
for any given patient) but each of the physicians has several patients.
(1) I am concerned about intra-physician correlation (non-independence of
observation violation) on the visits (possible correlation
between visits that occur to the same physician) and was
told that I could use either -xtgee- or -xtlogit, pa- to adjust for the
possibility of non-independence of observations due to the fact that each
physician had several patients. Is this true?
(2) If true, I am testing whether or not certain variables
contribute to the patient receiving a psychotherapeutic treatment (yes, no).
I am confused as to whether the 'i()' option should refer to the individual
visit
variable or to the individual physician variable or what. So I tried many
possibilities such as the following and all resulted in the error message :
estimates diverging (absolute correlation > 1)
r(430);
psyment is a 1,0 dichotomous variable; visitid is individual visit; phycoden
is individual physician
For example, I tried the following using 'i()' as the individual visit
variable
xi: xtgee psyment i.agecatc i.sex i.met i.GEOGREGN i.hmo i.patseen
i.mntlrecx
> i.anyment psydinum i.cadg i.physpecy psydrnum i.diagsrvcx i.anxdr
i.psycotdr
> i.depresdr i.miscnsdr i.ssricx i.phytimec i.insurtyp yearn if
csample==1,
> i(visitid) fam(bin) link(logit)
estimates diverging (absolute correlation > 1)
r(430);
then I tried a simpler model,
xtgee psyment psydinum psydrnum yearn if csample==1, i(visitid) fam(bin)
link(logit)
estimates diverging (absolute correlation > 1)
r(430);
then I switched to 'i()' referring to the individual physician variable
xtgee psyment psydin2 psydrnum yearn if csample==1, i(phycoden)
fam(binomial)
> link(logit) corr(exc) force
Iteration 1: tolerance = .54993564
Iteration 2: tolerance = .28833217
estimates diverging (absolute correlation > 1)
r(430);
then I tried xtlogit
. xtlogit psyment psydin2 psydrnum yearn if csample==1, i(phycoden) pa
Iteration 1: tolerance = .54993564
Iteration 2: tolerance = .28833217
estimates diverging (absolute correlation > 1)
r(430);
As you can see, none worked for me.
What am I doing wrong here?
Thanks
Wendell Joice
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