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st: RE: AW: Identifying unique values with codebook


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: AW: Identifying unique values with codebook
Date   Wed, 16 Jun 2010 15:13:08 +0100

As Martin says, at root this is a precision problem. 

Neither -codebook- nor anything else is to blame if it is presented with
the same values. To hold very large integers you may need to consider
-long- as another possibility, or even a string representation. 

Nick 
n.j.cox@durham.ac.uk 

Martin Weiss

As -help data_types- says: "doubles have 16 digits of accuracy." So you
can
increase the digits of your "y" up to the point where even -double- can
do
nothing for you:

*************
clear
set obs 10
gen byte x=_n
codebook x
gen double y1 = 1000000000000000 + x
gen double y2 = 10000000000000000 + x
gen double y3 = 100000000000000000 + x
gen double y4 = 1000000000000000000 + x
codebook y?
ins y?
*************


Interestingly, -inspect- seems to differ from -codebook-`s opinion.

Walter Garcia-Fontes

I stumped into a problem when identifying unique values of a numeric
variable using "codebook": if the values are large they will be
identified as the same value.  

For instance I have a variable x with the following values:
0, 1, 2, ... 10 (that is 10 different values)

codebook x 
reports "unique values: 10"

Now do 
gen y = 100000000000000000 + x

codebook y
reports "unique values = 1"

Is this a feature?

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