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RE: st: A strange data problem


From   "Nick Cox" <[email protected]>
To   <[email protected]>
Subject   RE: st: A strange data problem
Date   Mon, 17 Jan 2005 15:12:14 -0000

(This is second time round with this
problem, mostly likely through some 
oversight or list malfunction. I 
suggest the diagnosis is clear, 
so let's close the thread, please.) 

Nick 
[email protected] 

> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of 
> Ulrich Kohler
> Sent: 17 January 2005 14:44
> To: [email protected]
> Subject: Re: st: A strange data problem
> 
> 
> Maoyong Fan wrote:
> > 	I met a strange problem in dealing with my scanner 
> data.  I use stata
> > transfer to transger the original data to dta files.  One 
> of my variables
> > is UPC whose storage type is double.  I found that when I 
> generate a new
> > variable equal to UPC, the new variable is totally 
> different from UPC.
> >
> > Like this:
> > gen UPC1 = UPC
> > gen diff = UPC1 - UPC
> > sum
> > des
> >
> >     Variable |       Obs        Mean    Std. Dev.       Min 
>        Max
> > 
> -------------+--------------------------------------------------------
> >          UPC |   1780770    3.76e+10    1.96e+11   1.00e+10 
>   2.72e+12
> >         UPC1 |   1780770    3.76e+10    1.96e+11   1.00e+10 
>   2.72e+12
> >         diff |   1780770    144.9445    4290.722    -105114 
>     116086
> >
> 
> The default Storage type for new variables in Stata is float. 
> Float variables 
> have about 7 digits of accuracy; If the values in UPC have 
> more than 7 
> digits, these values have to be rounded to store them in a 
> float variable. 
> 
> To get an exact copy of your variable you need to generate 
> UPC1 with double 
> precision:
> 
> . gen double UPC1 = UPC
> 
> Note however, that the difference between  1.000.000.000 and 
> 1.000.000.144 is 
> probably not very important. Equally one is seldom interested in the 
> difference between numbers like 0,1234567144 and 0,1234567000. 
> 
> More on that can be found under the heading "Precision of 
> numeric storage 
> types" in help datatypes and in [U] 16.10
> 

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