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If Richard's suggestion of using option -difficult- doesn't solve your problem,
then consider the following.
1. Confirm that Stata's -xtlogit- and SAS's PROC LOGISTIC are seeing the same
dataset. One way to verify that the data each sees are the same is to compare
the log-likelihoods. Because you cannot get convergence with Stata when the
predictors are included, fit a model with no predictors (-clogit Y, group(ID)-).
Then compare twice the negative log-likelihood value from Stata with null-model
value shown by SAS (you mention that it's 3930972). Are they identical? If
not, then there's probably a data-management error causing a difference in the
two datasets. With the size of your dataset, this might not be particularly
sensitive to occasional differences, but it will detect systematic differences.
(It might even not be very specific; the log-likelihoods might differ despite an
identical dataset, which would be helpful to know, too: see the footnote to 2.
below.) You've already compared a subset of your dataset, and the results
match, but there could be some systematic difference in the longer panels.
Also, verify that the number of singletons (and other cases with a constant
response) that is being thrown out by both packages is the same. Stata gives
you a message before the iteration begins ("note: XXX groups (YYY obs) dropped
because of all positive or all negative outcomes.". SAS says "Number of
Uninformative Strata" and "Frequency Uninformative". Both numbers should agree
2. If Stata's Marquardt algorithm isn't so aggressive as SAS's (or Stata's
singularity threshold is too sensitive), then you can try to side-step the
Hessian altogether. Try -clogit . . . , . . . technique(bhhh)-. (See
http://www.stata.com/statalist/archive/2010-03/msg01192.html for a thread by
someone with the same problem as you report.*)
3. If that fails, then you can go the route that SAS used to use for
conditional/fixed-effects logistic regression prior to the STRATA statement,
namely, Cox regression.
generate byte time = 2 - Y
stset time, failure(Y = 1)
stcox DUM CONT1 CONT2, strata(ID) exactp nohr
4. If everything fails, then you might need to use SAS's answer, as Klaus
suggests. In light of the warnings from Stata, you might want to check on a
couple of things in SAS's model-fit before relying extensively on it.
a. You mention two lines in your SAS output.
Newton-Raphson Ridge Optimization
Without Parameter Scaling"
The very next line in the output, the one just after that last line above. You
didn't mention it. Does it say, "Convergence criterion (GCONV=1E-8)
satisfied."? The same claim should be repeated in the SAS .LOG file.
b. Is everything else agreeable in the SAS .LOG file?
c. You mentioned that the omnibus tests are all P < 0.0001. What do the
regression coefficients and their covariance matrix look like? Are they
d. You probably didn't ask for an iteration trace in the SAS run, but it would
be good to see how things look at convergence. I haven't tried the following
for a run that blows up, but I believe that you can get an idea of SAS's
gradient and Hessian at-convergence by feeding its regression coefficients to
Stata and then not iterating at all. If it works, then it avoids re-running the
model-fit in SAS. Try the steps below.
Type in SAS's logit (untransformed) regression coefficients at full displayed
precision into a Stata matrix.
matrix input Beta = (<DUM's coefficient> <CONT1's coefficient> ///
clogit Y DUM CONT1 CONT2, group(ID) from(Beta, copy) ///
iterate(0) gradient hessian
Are you satisfied that the gradient's length is reasonably close to zero, that
SAS's GCONV was tight enough? Which predictor is Stata complaining about in the
Hessian? (Probably DUM, from your description of the dataset.) Look back at
4.c. above, again, asking how sensible that predictor's coefficient and standard
*That the same problem arose twice in independent situations, combined with your
observation that another software package has no trouble, raises the distinct
possibility that there's a bug in Stata's -clogit-. If so, then it's a rare bug
that's difficult for StataCorp to replicate and fix without help from users.
I'm obviously just guessing here, but from the user's manual, the objective
function that -clogit- maximizes resembles a penalized log-likelihood, and
something like a problem in the recursive algorithm to compute the conditioning
factor looks as if it could give rise to the kind of behavior you and Yu Xue
describe. Regardless, if you're satisfied with the items in 4. above, then you
might be doing everyone a favor by contacting StataCorp for follow-up.
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