Re: st: Re: logical missing as true

[David Kantor <dkantor@jhu.edu>]

At 04:38 PM 1/12/2004 +0000, Paul Seed wrote:
> Some more thoughts:
>
> As we have been reminded by Alan Reese & Nick Cox
> Stata  consistently treats missing as true.
>
> By contrast, all given SPSS's logical expressions are
> correct if "missing" is read as "false".  Moreover, in their own words:"The
> assignment is executed only if the logical expression is true."
>
> However, I am not sure how SPSS evaluates "not missing".
> I would expect SPSS to treat it as "missing".
>
> If we are to change this, I would like two things:
>
> A warning when missing values are evaluated in a logical expression
>
> An option to reset the logical value of "missing" to either true (default),
> false, or error.  If error is selected, evaluating logical expressions
> including missing values will generate  an error code, and (usually)
> stop execution of code.
>
> If available, I would probably set missing = error in most programs,
> as it is is almost always a mistake when I do not error-trap the
> missing values.
>
> Whatever else StataCorp decide to do, I would expect the current
Yet more thoughts:

I recall conversing with Bill Gould two years ago on this matter.  He had 
an interesting idea, advocating the concepts of "definitely true"  and 
"definitely false" in these situations.  Under this scheme, missing would 
be neither true nor false.  Thus, in...

if exp {
 ...
}
else {
 ...
}

if exp were missing, then neither the -if- nor the -else- part would get 
executed.  (Maybe there could be a third section -- if-else-miss 
?)  Similarly, ...

do_something if exp

would include only those cases that are definitely true (equivalent to ...
 do_something if exp & ~mi(exp)
under current Stata semantics), and

do_something if ~exp

would apply to only those that are definitely false (numeric 0).  Again, 
those cases that evaluated to missing would be excluded from both of the 
above statements.

Currently, ~. is 0.  The proposed scheme would require making ~. = 
.   i.e., if an expression is missing, then so is its negation.

I hope this gets some of us thinking.
-- David

David Kantor
Institute for Policy Studies
Johns Hopkins University
dkantor@jhu.edu
410-516-5404

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