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AW: st: AW: Diffrence between sum and tabsum


From   "Martin Weiss" <martin.weiss1@gmx.de>
To   <statalist@hsphsun2.harvard.edu>
Subject   AW: st: AW: Diffrence between sum and tabsum
Date   Tue, 15 Sep 2009 13:45:32 +0200

<> 

Also note http://www.stata.com/support/faqs/data/float.html

I get answers different from yours, both in Stata 10.1 and 11, possibly b/c your dataset is not created by the code that you showed in your reply? I assume this is the case as your -list- command shows the "dummy" although it is -generate-d in the following line... 



HTH
Martin


-----Ursprüngliche Nachricht-----
Von: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] Im Auftrag von Ulrich Kohler
Gesendet: Dienstag, 15. September 2009 13:33
An: statalist@hsphsun2.harvard.edu
Betreff: Re: st: AW: Diffrence between sum and tabsum

<>

yes, I thought about that. However, the following bit shows that your
example is not quite the problem here

. set obs 3
. gen byte coreweight1968=18
. gen double firstweight =   coreweight1968 *  60468000/78028
. format firstweight %32.20f
. l

     +-----------------------------------------------+
     | cor~1968                  firstweight   dummy |
     |-----------------------------------------------|
  1. |       18   13949.14646024503963417374       1 |
  2. |       18   13949.14646024503963417374       1 |
  3. |       18   13949.14646024503963417374       1 |
     +-----------------------------------------------+

. gen dummy = 1
. tab dummy, sum(firstweight)

            |       Summary of firstweight
      dummy |        Mean   Std. Dev.       Freq.
------------+------------------------------------
          1 |   13949.146   .00024815           3
------------+------------------------------------
      Total |   13949.146   .00024815           3


However you are right that is should be some kind of a precision
problem. In fact 

. replace firstweight =   coreweight1968 *  4/3
. tab dummy, sum(firstweight)

works as expected.


Am Dienstag, den 15.09.2009, 12:54 +0200 schrieb Martin Weiss:
> <> 
> 
> 
> Probably a precision issue as in
> 
> ***
> 
> clear*
> 
> inp double firstweight
> 13949.1460454864
> 13949.146248765 
> 13949.146324324
> end
> 
> 
> gen byte dummy=1
> 
> su firstweight
> 
> tab dummy,  /* 
> */ sum(firstweight)
> 
> ***
> 
> 
> 
> HTH
> Martin
> 
> -----Ursprüngliche Nachricht-----
> Von: owner-statalist@hsphsun2.harvard.edu
> [mailto:owner-statalist@hsphsun2.harvard.edu] Im Auftrag von Ulrich Kohler
> Gesendet: Dienstag, 15. September 2009 12:14
> An: statalist
> Betreff: st: Diffrence between sum and tabsum
> 
> <>
> 
> Does anybody know why -tabulate, summarize- displays a nonzero
> standard-deviation for a variable that is a constant? 
> 
> . tab seopsu if seopsu==948, sum(firstweight)
> 
>      Cps Pr |
>    Sampl Un |       Summary of firstweight
>     5:71-73 |        Mean   Std. Dev.       Freq.
> ------------+------------------------------------
>         948 |   13949.146   .00024815           3
> ------------+------------------------------------
>       Total |   13949.146   .00024815           3
> 
> . sum firstweight if seopsu==948
> 
>     Variable |       Obs        Mean    Std. Dev.       Min        Max
> -------------+--------------------------------------------------------
>  firstweight |         3    13949.15           0   13949.15   13949.15
> 
> . return list
> 
> scalars:
>                   r(N) =  3
>               r(sum_w) =  3
>                r(mean) =  13949.14646024504
>                 r(Var) =  0
>                  r(sd) =  0
>                 r(min) =  13949.14646024504
>                 r(max) =  13949.14646024504
>                 r(sum) =  41847.43938073512
> 
> . d firstweight 
> 
>               storage  display     value
> variable name   type   format      label      variable label
> ----------------------------------------------------------------------------
> ------
> firstweight     double %10.0g                 
> 
> . l firstweight if seopsu==948
> 
>       +-----------+
>       | firstwe~t |
>       |-----------|
> 4256. | 13949.146 |
> 4365. | 13949.146 |
> 4537. | 13949.146 |
>       +-----------+
> 
> . about
> 
> Stata/MP 11.0 for Unix (Linux 64-bit x86-64)
> Born 26 Aug 2009
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> 
> 
> 
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