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Re: st: Questions on the rules guiding the -table- command


From   Sergiy Radyakin <serjradyakin@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Questions on the rules guiding the -table- command
Date   Wed, 29 Aug 2012 11:22:29 -0400

Dear Clyde, thank you for your comments. Best, Sergiy

On Wed, Aug 29, 2012 at 11:00 AM, Clyde B Schechter
<clyde.schechter@einstein.yu.edu> wrote:
> Sergiy Radyakin asks, among other things:
>
>  First, consider the following example:
>
> version 12.0
> clear all
> webuse nlsw88
>
> // does not include the "mining" category
> table industry union,c(mean wage) row, if union==1
>
> // does include the "mining" category
> generate unionwage=wage if union==1
> table industry union, c(mean unionwage) row
>
> // does not include the "mining" category
> tabulate industry union if union==1
>
> In the results you may see that some of the tables include the "mining" category
> and some don't. I would like to learn more about the rules of inclusion and the
> background if possible.
>
> I think the behavior he observes has nothing to do with the -table- command, but reflects the behavior of -if- conditions.
>
> The manual says "The if exp qualifier restricts the scope of a command to those observations for which the value
> of the expression is true."  [U] 11.1.3.
>
> Otherwise put,
>
> -<command> if <condition>-
>
> is equivalent to
>
> preserve
> keep if <condition>
> -<command>-
> restore
>
> In the nlsw88 data set, there are _no_ observations where industry == "Mining" and union == 1:
>
>
> . tab industry union
>
>                       |     union worker
>              industry |  nonunion      union |     Total
> ----------------------+----------------------+----------
> Ag/Forestry/Fisheries |        10          2 |        12
>                Mining |         2          0 |         2
>          Construction |        17          3 |        20
>         Manufacturing |       234         84 |       318
> Transport/Comm/Utilit |        39         48 |        87
> Wholesale/Retail Trad |       239         21 |       260
> Finance/Ins/Real Esta |       144          9 |       153
>   Business/Repair Svc |        51          8 |        59
>     Personal Services |        58          5 |        63
> Entertainment/Rec Svc |        12          2 |        14
> Professional Services |       500        220 |       720
> Public Administration |       101         56 |       157
> ----------------------+----------------------+----------
>                 Total |     1,407        458 |     1,865
>
>
> So when a command is issued with -if union == 1-, the command is carried out on a (virtual) data set in which there are no observations where industry is Mining.  So Mining will not appear in the output of that command (nor be involved in its computations), any more than would, say, "Health Care" which doesn't appear anywhere in the data set at all.
>
> By contrast, the command that produced output which did include the Mining category has no -if- condition, so -table- is applied to the entire data set, including those observations where industry is Mining, notwithstanding the absence of any values for the variable unionwage.
>
> His second question about the interpretation of the Total row, I believe is similarly explained by the way -if- operates.
>
> His other question I cannot answer--I think only those who authored the -table- command would know.
>
> Hope this helps.
>
> Clyde Schechter
> Department of Family & Social Medicine
> Albert Einstein College of Medicine
> Bronx, New York, USA
>
>
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