st: Missing value in -merge- output but not in _merge variable

[Bert Jung <bjung59@gmail.com>]

Dear Statalisters,

I am trying to understand the following output from a 1:1 merge in
Stata 11.2.  The merge output shows a missing value for _merge==3 but
a tab shows observations with _merge==3.


(There are no other _merge variables in the two datasets.)

. merge 1:1 hhid pid using test.dta

    Result                           # of obs.
    -----------------------------------------
    not matched                        21,164
        from master                    20,260  (_merge==1)
        from using                        904  (_merge==2)

    matched                                 .  (_merge==3)
    -----------------------------------------

. tab _merge

                 _merge |      Freq.     Percent        Cum.
------------------------+-----------------------------------
        master only (1) |     20,260       60.26       60.26
         using only (2) |        904        2.69       62.95
            matched (3) |     12,455       37.05      100.00
------------------------+-----------------------------------
                  Total |     33,619      100.00



A similar question was raised in
http://www.stata.com/statalist/archive/2010-11/msg00661.html.  That
question remained unanswered and I have been unable to find an
explanation on the list or in the manuals.

Any thoughts warmly welcome.

Thanks, Bert
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