st: stcox: weighted regression changes the # of observations

[Michael McCulloch <mm@pinest.org>]

(I'm sorry for the re-send, my last note had poor column alignment.)
I'd appreciate your consideration of this question.
I'm conducting cox regression on a dataset measuring survival after 
treatment. I'm puzzled as to why weighted regression (using   -iweight- 
after -stcox-) would show a different # of observations in the output. Here 
are results of the ways I've done the analysis. In each case, I start with 
only 33 subjects.

WEIGHTED STCOX
*bootstrap, reps(1000): stcox tcm
	HR=0.72 (se 0.26), ci95(0.36, 1.48)   "No subjects" = 65.62

UN-WEIGHTED STCOX
stcox tcm
	HR = 0.77 (se 0.30), ci95 (0.36, 1.66)	   "No subjects" = 33

parmby "stcox tcm "
	HR = 0.77 (se 0.30), ci95(0.36, 1.66)   "No subjects" = 33
	
parmby "bootstrap, reps(1000): stcox tcm
	HR = 0.77 (se 0.45), ci95(0.40, 1.49)   "No subjects" = 33

bootstrap, reps(1000): stcox tcm
	HR = 0.77 (se 0.26), ci95(0.40, 1.50)   "No subjects" = 33

*Note: I'm using inverse probability of treatment weighting, with the 
following command immediately preceding -stcox-:
stset datedied [iweight=wt_unst_II]

Another anomaly is why the bootstrap option yields a narrower ci95.






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