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RE: st: File sizes in Stata & SPSS (was Weights )


From   "Steichen, Thomas J." <SteichT@rjrt.com>
To   "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu>
Subject   RE: st: File sizes in Stata & SPSS (was Weights )
Date   Fri, 2 May 2008 13:21:46 -0400

A follow-up clarification...

In the instance where a new variable being encoded does not have some of the values already coded by the named value label, the option will not remove unused values from the value label; it will only add additional values if needed.

-----------------------------------
Thomas J. Steichen
steicht@rjrt.com
-----------------------------------

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Steichen, Thomas J.
Sent: Friday, May 02, 2008 12:42 PM
To: 'statalist@hsphsun2.harvard.edu'
Subject: RE: st: File sizes in Stata & SPSS (was Weights )

Peter,

It's called the label() option of encode.

The encode help file reads (in part):

label(name) specifies the name of the value label to be created or used and added
        to if the named value label already exists.  If label() is not specified,
        encode uses the same name for the label as it does for the new variable.

Tom

-----------------------------------
Thomas J. Steichen
steicht@rjrt.com
-----------------------------------

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Lachenbruch, Peter
Sent: Friday, May 02, 2008 11:36 AM
To: statalist@hsphsun2.harvard.edu
Subject: RE: st: File sizes in Stata & SPSS (was Weights )

If you decode and then encode again to get small files, your encoded
values may not be than same from data set to data set.  Perhaps one way
to do this is to modify the label definitions (potentially a real pain
in the neck).  Maybe someone brighter than me can come up with a simple
do file for this:  detect the unique values and retain the label
definitions for them.

Tony

Peter A. Lachenbruch
Department of Public Health
Oregon State University
Corvallis, OR 97330
Phone: 541-737-3832
FAX: 541-737-4001

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of David Kantor
Sent: Friday, May 02, 2008 7:47 AM
To: statalist@hsphsun2.harvard.edu
Subject: RE: st: File sizes in Stata & SPSS (was Weights )

Hello all,

I just want to add some observations about encoding.

When you encode a string variable, the file contains a copy of every
distinct value. Consequently, it provides a space advantage usually
only if many of the values are repeated. If all or most observations
are distinct, then encoding will not gain a space advantage. (But you
may have other reasons for encoding.)

But even when encoding is advantageous in terms of space, there is
one situation when it can backfire; I had not though of this until it
happened to me. I had a large file with a string variable with many
distinct values -- though many were often repeated. I encoded it, and
gained a significant space savings.

Later, I created a multitude of smaller subsets of this file. Each
one had much fewer distinct values of the encoded variable. But each
file retained the full encoding table -- more than it needed. (Each
file replicated the encoding table.) The result was that each of the
small files were much bigger than they really needed to be. (And the
total size may have been much more then the original, even if there
had been no overlap of observations.) Subsequently, I decoded the
variable, and the files shrunk significantly.

I thought this is something to be aware of.
(It makes a potential case for having coding tables in a separate
file. But there are plenty of reasons not to have it that way.)

--David

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