I cannot comment on STATA. In Stata, numeric missing counts
as very large, so
drop if x > 450
will -drop- all observations on which -x- is missing.
(Note that the test
... if x != .
does _not_ exclude any of .a ... .z.)
This is an FAQ:
FAQ . . . . . . . . . . . . . . . . Logical expressions and missing values
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . W. Gould
2/03 Why is x > 1000 true when x contains missing value?
http://www.stata.com/support/faqs/data/values.html
Running regressions is a different matter. -regress-, for example, ignores observations
with missing values, but it does _not_ -drop- them.
Nick
[email protected]
[email protected]
> when I use the following command:
> drop if x>450
> STATA drops a lot of observations, while when I exclude
> missing values as
> follows:
> drop if x>450 & x!=.
> STATA eliminates just a couple of observations
> I realized this when I run a regression including x as
> regressor. If STATA
> drops missing data with the first command, shouldn't drop the same
> observations when I run the regression after using the second command?
> why doesn't it happen?
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