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st: RE: Why does Stata do it wrong?


From   Joly.Patrick@ic.gc.ca
To   statalist@hsphsun2.harvard.edu
Subject   st: RE: Why does Stata do it wrong?
Date   Mon, 9 Jun 2003 09:58:43 -0400

Tesfayi Gebre wrote
> I want to generate a single unique id to merge files
> by using variables: var1 (F1) var2 (F3) var3 (F4) and
> var4 (F2), where F shows the integer width (f format).
> So, I used 
> 
> gen id = var1*10^9 + var2*10^6 + var3*10^2 + var4
> 
> This is the output from stata and the true id (as it
> should be) for the following
> example:
> 
> var1   var2   var3  var4   id (stata)   id (true)
> 1      2      3452   1     1005452032	1002345201
> 1      121    34     10	   1121033984	1121003410
> 2      23     156    2     2023155968	2023015602
> 2      45     3      2     2045003008	2045000302
> 3      6      4      3     3006003968	3006000403
> 3      70     8      5     3070008064	3070000805
> 
> Why is the stata id different from the true id? Does
> stata store numbers in different format?

Generate your id variable as a double numerical type.  Doubles have up to 16
digits of accuracy, i.e.

   gen double id = var1*10^9 + var2*10^6 + var3*10^2 + var4

I presume your default datatype is -float- so values were not accurately
rendered when you generated id.  Floats only have approx 7.22 digits of
accuracy.  See http://www.stata.com/support/faqs/data/prec.html and -help
datatype- for more info.


Patrick Joly
joly.patrick@ic.gc.ca
pat.joly@utoronto.ca
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