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Re: st: the -missing- command


From   "louis boakye-yiadom" <louisby@hotmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: the -missing- command
Date   Tue, 11 Oct 2005 12:02:50 +0000

Thanks, Jeff and Michael, for the very helpful replies.

Louis

From: jpitblado@stata.com (Jeff Pitblado, StataCorp LP)
louis boakye-yiadom <louisby@hotmail.com> asks about the -missing()- function:

> I have two questions regarding the -missing- command (or function?).

> First question:
> I'm not clear about exactly what -missing- does. An example will clarify my
> concern. I have a dataset containing 11 variables (the first is -clust- and
> the last is -s11aq13-). One of the variables is -trexpcd-, and the total
> number of observations is 4872. I gave the following commands and had the
> shown output:

> count if mi(trexpcd)
> 4649
> count if mi(clust-s11aq13)
> 82

> My understanding of the online -help- is that -missing- evaluates the number
> of observations for which any of the arguments is missing. So, for the
> second command, since -trexpcd- is one of the arguments, I expected the
> result to be a number which is at least equal to 4649.

> Can anyone please show me what I'm missing?

> Second question:
> Also, I had the following result:
> count if !mi(clust-s11aq13)
> 4790

> Does !mi() evaluate the number of observations for which none of the
> arguments is missing? Thank you.

I believe what Louis wants is Nick Cox's -nmissing- command, which (among
other things) reports the number of missing values found in the variable of
the dataset:

. ssc install nmissing
(output omitted)

. sysuse auto
(1978 Automobile Data)

. nmissing

rep78 5

.

There is also a companion command called -npresent- that reports the number of
non-missing values found in each variable of the dataset:

. npresent

make 74
price 74
mpg 74
rep78 69
headroom 74
trunk 74
weight 74
length 74
turn 74
displacement 74
gear_ratio 74
foreign 74

Also, look into the -mark- and -markout- commands. They help you generate a
variable that indicates observations that are free of missing values for a
specified list of variables:

. mark touse

. markout touse price-foreign

. count if touse
69

Svend Juul <SJ@SOCI.AU.DK> also replied and was puzzled by the results of the
following commands:

. count if missing(rep78-headroom)
5

. count if missing(mpg-headroom)
0

-missing()- is a function, in this context "rep78-headroom" is interpreted
to mean the difference between the variables -rep78- and -headroom-.

--Jeff
jpitblado@stata.com

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