st: Treatment of missing values in surveys in Stata (subpop)

[Ángel Rodríguez Laso <angelrlaso@gmail.com>]

Dear Statalisters,

I have found two recommended procedures for dealing with individuals
with missing items ('normal' missing answers like 'DK/DA' or equipment
failure) when analysing surveys with Stata:

1) One is based on the recommendation that, unless there is a very
strong reason to do otherwise, whenever you analyse a group of
individuals in a survey with Stata, you have to use subpop. (See for
example: http://www.stata.com/meeting/mexico10/mex10sug_canette.pdf).
Under this perspective, those with valid values would be a
subpopulation. From my point of view, this means that in order to
prevent Stata from dropping them from the calculation of standard
errors, missing codes (".") should be recoded to a numerical value
(like 999) and then a command issued this way:

svy, subpop(if var<999): command var

2) Nevertheless, most of the information I've read does not make any
statement about this, what implicitly means that missing codes don't
need to be recoded. I've even found this piece of advice
(http://www.stata.com/statalist/archive/2012-09/msg01028.html): 'I've
never seen a recommendation to consider observations with non-missing
values as a subpopulation'


I wonder if anyone could throw some ligth on this topic.

Thank you very much.

Angel Rodriguez-Laso
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