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st: RE: RE: RE: generating a new variable with the egen command


From   "Maarten Buis" <M.Buis@fsw.vu.nl>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: RE: RE: generating a new variable with the egen command
Date   Mon, 28 Nov 2005 17:20:20 +0100

Since precision came up again, I would like to add a comment:

While it is good practice to try to minimize rounding errors during computation (e.g. during computing new variables that are sums), you should keep in mind how precise your measurement on that variable actually is. For instance, I teach introductory statistics to first year social science students. In the Netherlands grades run from 0 (didn't even spell their own name right) to 10 (brilliant). Each year at least one of them asks whether I would want to give them grades with two decimal points accuracy. I think I make good exams, but they cannot distinguish between a student with a statistics capability worth a 6.01 and worth a 6.02. 

Eight digits accurate (float) should be more than enough for most measurements; in most real data I would consider sixteen digits (double) overkill.

HTH,
Maarten 

-----------------------------------------
Maarten L. Buis
Department of Social Research Methodology 
Vrije Universiteit Amsterdam 
Boelelaan 1081 
1081 HV Amsterdam 
The Netherlands

visiting adress:
Buitenveldertselaan 3 (Metropolitan), room Z214 

+31 20 5986715

http://home.fsw.vu.nl/m.buis/
-----------------------------------------

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu]On Behalf Of Maarten Buis
Sent: maandag 28 november 2005 16:57
To: statalist@hsphsun2.harvard.edu
Subject: st: RE: RE: generating a new variable with the egen command

Furthermore, calculating the sums in float format isn't as bad as it initially seems: the fact that Stata shows only 3 digits of precision has to do with the display format. The float is precise up to 8 digits, and is stored that way, as can be seen in the program below.

*-------------------------example------------------
drop _all
input x
        42224464
      67090781
end

egen sum = sum(x)
egen long sumlong = sum(x)

list sum sumlong

format sum %20.7g

list sum sumlong
*------------end example-----------------

HTH,
Maarten


-----------------------------------------
Maarten L. Buis
Department of Social Research Methodology
Vrije Universiteit Amsterdam
Boelelaan 1081
1081 HV Amsterdam
The Netherlands

visiting adress:
Buitenveldertselaan 3 (Metropolitan), room Z214

+31 20 5986715

http://home.fsw.vu.nl/m.buis/
-----------------------------------------

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu]On Behalf Of Nick Cox
Sent: maandag 28 november 2005 16:42
To: statalist@hsphsun2.harvard.edu
Subject: st: RE: generating a new variable with the egen command

Your call should be

egen double newvar = ...

or

egen long newvar = ...

If these are integers, -long- is better. Either type
is allowed by the syntax. See the help for -egen- once more.

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

>agostino@unical.it
>
> I'm trying to generate a new variable given by the sum of an
> existing variable
> by year (as I have 2 six month observations for the same
> year) and individual.
> The command is the following:
> egen newvar=sum (oldvar), by(id year)
>
> the problem is that the original values are, for instance:
> 42224464 and
> 67090781, but the sum that STATA computes is 109000000,
> instead of 109315245.
> Why does it round like that?
> The old variable is stored as "double", I have tried to
> change the storage
> type, using recast, but STATA does not allow to do it, as
> many values would be
> changed...



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