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------------------------ | condition time | 0 1 ----------+------------- 0 | 1 1.202 1 | 2.07 .854 ------------------------ The means of observed cell counts are below: ---------------------------- | condition time | 0 1 ----------+----------------- 0 | 2.09559 2.64706 1 | 4.36765 1.91176 ---------------------------- These are in the ratios below, which differ from the predicted ratios: ------------------------ | condition time | 0 1 ----------+------------- 0 | 1 1.263 1 | 2.084 .912 ------------------------ I had expected that the observed and predicted relative cell frequencies would be the same for the fitted model. Can anyone explain why they are not? Question 2. I believed that variables which are constant within ID could not be used in a fixed effects model. Am I wrong? It seems that I am, as illustrated by the inclusion of diagnosis (which is constant within a subject). . xi: xtnbreg count diagnosis i.time*condition, i(id) fe irr nolog i.time _Itime_0-1 (naturally coded; _Itime_0 omitted) i.time*condit~n _ItimXcondi_# (coded as above) Conditional FE negative binomial regression Number of obs = 544 Group variable (i): id Number of groups = 136 Obs per group: min = 4 avg = 4.0 max = 4 Wald chi2(4) = 172.78 Log likelihood = -643.28819 Prob > chi2 = 0.0000 ------------------------------------------------------------------------------ count | IRR Std. Err. z P>|z| [95% Conf. Interval] -------------+---------------------------------------------------------------- diagnosis | 7.784788 5.952364 2.68 0.007 1.739424 34.8408 _Itime_1 | 2.072968 .1601387 9.44 0.000 1.781708 2.411841 condition | 1.180407 .1038162 1.89 0.059 .9935027 1.402473 _ItimXcond~1 | .3444533 .0414192 -8.86 0.000 .2721301 .4359976 ------------------------------------------------------------------------------ Question 3 Can anyone offer advice as to how to assess goodness of fit for xtnbreg ... , fe? (some models I want to fit are rather more complex than the above examples). Any advice or comments will be appreciated Thanks John Plummer * * 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/

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