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From |
"Wendell Joice" <wjoice@erols.com> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
st: uting for Non-independent observations using -xtgee- or -xtlogit-, pa??? |

Date |
Sun, 2 Mar 2003 20:12:04 -0500 |

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 * * 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/

**Follow-Ups**:**Re: st: uting for Non-independent observations using -xtgee- or -xtlogit-, pa???***From:*Ricardo Ovaldia <ovaldia@yahoo.com>

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