Re: st: question on cond( )

[Nick Winter <nwinter@virginia.edu>]

It looks to me like the examples in the help for cond() are either 
incorrect or misleading.

The function cond(condition,a,b,c)

returns -a- if -condition- is true; -b- if -condition- is false, and -c- 
if -condition- is missing.

Note that last is "the *condition* is missing"; that is, that the 
statement evaluates to missing.  This is *not* the same as some part of 
-condition- evaluating to missing.

So in the example where condition is "x>2", this condition evaluates to 
either true or false for all observations, including cases where x=., 
because the condition ".>2" is true under Stata's handling of missing 
values.

This seems to make the following statement from the help file wrong: 
"cond(a>2,"this","that","missing") = "missing" if a > ."

The only way I can think of to trigger the "missing" option would be 
something like this:

clear
set obs 10
gen x=_n-1 in 1/8
gen z=cond(x,"true","false","missing")
list

     +-------------+
     | x         z |
     |-------------|
  1. | 0     false |
  2. | 1      true |
  3. | 2      true |
  4. | 3      true |
  5. | 4      true |
     |-------------|
  6. | 5      true |
  7. | 6      true |
  8. | 7      true |
  9. | .   missing |
 10. | .   missing |
     +-------------+

But once you are doing a comparison (x>2), that will always evaluate to 
either "true" or "false" in Stata; never to missing.

--Nick Winter

Visintainer, Paul wrote:

> I'm not sure why the "condition" function is not coding z with 2 missing
> values.  If I'm reading the functions command correctly, z should be
> coded as missing:          
> cond(a>2,"this","that","missing") = "missing" if a > .
> cond(a>2,"this","that","missing") = "this" if a > 2 and a < .
>
> Any ideas?
>
> Thanks.
>
> . gen z=cond(x>5,1,0,.)
>
> . list
>
>      +-------+
>      | x   z |
>      |-------|
>   1. | 1   0 |
>   2. | 2   0 |
>   3. | 3   0 |
>   4. | 4   0 |
>   5. | 5   0 |
>      |-------|
>   6. | 6   1 |
>   7. | 7   1 |
>   8. | 8   1 |
>   9. | .   1 |
>  10. | .   1 |
>      +-------+
>
>
>
> ______________________________________
> Paul F. Visintainer, PhD
>
>
> *
> *   For searches and help try:
> *   http://www.stata.com/support/faqs/res/findit.html
> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/
--
--------------------------------------------------------------
Nicholas Winter                                 434.924.6994 t
Assistant Professor                             434.924.3359 f
Department of Politics                  nwinter@virginia.edu e
University of Virginia          faculty.virginia.edu/nwinter w
PO Box 400787, 100 Cabell Hall
Charlottesville, VA 22904

*
*   For searches and help try:
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