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From |
Nick Winter <nwinter@virginia.edu> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: question on cond( ) |

Date |
Thu, 28 Feb 2008 12:11:17 -0500 |

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: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**RE: st: question on cond( )***From:*"Nick Cox" <n.j.cox@durham.ac.uk>

**References**:**st: question on cond( )***From:*"Visintainer, Paul" <PAUL_VISINTAINER@NYMC.EDU>

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