st: dropped cases using mi impute mvn

[exp12 <exp12@psu.edu>]

I have previously used ice, but moved to mi impute as all of my students now
have access to Stata 11.

As a test run, a did a simple imputation on a survey data with 2812 records,
seven variables with missing data, and two variables with complete data. 
One of the seven variables is qualitative and represented by two dummy
variables in the imputation (with original and omitted category treated as
passive).

But the imputed data sets all have fewer records than the original.  Any
explanation for why cases are being lost would be welcome.

The relevant lines of code are:

mi set mlong
mi register imputed y1 y2 y3 y4 y5 y6 dummy1 dummy2
mi register regular x1 x2 
mi register passive qualvar dummy0
mi impute mvn y1 y2 y3 y4 y5 y6  dummy1 dummy2 = x1 x2, add(3)

This runs without error, but as you can see below, the number of cases is
wrong for the three imputed data sets.  Somehow 730 cases are dropped.

.	tab	_mi_m

		_mi_m       Freq.	Percent	Cum.
					
		0       2,812	31.04	       31.04
		1       2,082	22.99	       54.03
		2       2,082	22.99        77.01
		3       2,082	22.99	      100.00
					
	Total         9,058	       100.00

Any explanations? Suggestions?

Thanks,
Eric


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