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From |
"Nick Cox" <n.j.cox@durham.ac.uk> |

To |
<statalist@hsphsun2.harvard.edu> |

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

Date |
Thu, 28 Feb 2008 18:02:19 -0000 |

When David Kantor and I wrote a tutorial on -cond()- in SJ-5-3 pr0016 . . Depending on conditions: a tutorial on the cond() function . . . . . . . . . . . . . . . . . . . . . . . D. Kantor and N. J. Cox Q3/05 SJ 5(3):413--420 (no commands) tutorial on the cond() function we didn't even get to the four argument case. I agree with Nick Winter that the on-line help for the four argument case looks wrong. I think what Paul wants is well (if not best) coded like this: gen z = cond(missing(x), ., x > 5) This has all of a sudden come to be my favourite way to code creation of dummy/dichotomous/binary/logical/quantal/Boolean variables that could be 1, 0 or missing. (Any other synonyms?) It's perhaps simpler at first sight to code like this gen z = x > 5 if x < . in which the mapping of missings to missings is tacit, that is, if x is missing Stata does not use the result of (x > 5) but assigns missings. But then if you have some more complicated definition involving two or more variables you have to trap all the problems on all the variables: gen z = x > y if x < . & y < . This could be gen z = x > y if !missing(x, y) but as said I like to turn it round gen z = cond(missing(x, y), ., (x > y)) That way it's explicit what happens with missings. And it's quite easy to put in words: If there are missings on any x or y, return missing; otherwise evaluate (x > y). Yet more variables can be packed into the -missing()-: gen z = cond(missing(x, y, a, b), ., (x > y) & (a == b)) In all the above, -gen byte z- rather than -gen z- is careful on storage. Nick n.j.cox@durham.ac.uk Nick Winter 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. Visintainer, Paul > 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 | > +-------+ * * 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:*"Visintainer, Paul" <PAUL_VISINTAINER@NYMC.EDU>

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

**Re: st: question on cond( )***From:*Nick Winter <nwinter@virginia.edu>

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