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Re: st: is something wrong with fvvarlist "i(3 4).role"?


From   "Michael N. Mitchell" <Michael.Norman.Mitchell@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: is something wrong with fvvarlist "i(3 4).role"?
Date   Mon, 12 Jul 2010 18:56:12 -0700

Dear Lazlo

I think that this is more intuitive when thinking about using the -i.- prefix in the context of a modeling command, for example...

regress mpg i.rep78

In this situation, we always need a base category for the indicator variables. While a command like -summarize- supports factor variables, the default behavior is still to create a reference category. But you can override this. If you want to select all categories, you can do so like this...

. summ ibn.rep78

    Variable |       Obs        Mean    Std. Dev.       Min        Max
-------------+--------------------------------------------------------
       rep78 |
          1  |        69    .0289855    .1689948          0          1
          2  |        69     .115942    .3225009          0          1
          3  |        69    .4347826    .4993602          0          1
          4  |        69    .2608696    .4423259          0          1
          5  |        69    .1594203    .3687494          0          1

but if you want to select a subset of categories and have no base category, it looks like you would need to use the -fvset- command, for example

. fvset base none rep78
. summ i(1 2).rep78

    Variable |       Obs        Mean    Std. Dev.       Min        Max
-------------+--------------------------------------------------------
       rep78 |
          1  |        69    .0289855    .1689948          0          1
          2  |        69     .115942    .3225009          0          1

I hope this helps this make more sense. It is definitely a different way of thinking about specifying the levels of variables.

Best regards,

Michael N. Mitchell
Data Management Using Stata      - http://www.stata.com/bookstore/dmus.html
A Visual Guide to Stata Graphics - http://www.stata.com/bookstore/vgsg.html
Stata tidbit of the week         - http://www.MichaelNormanMitchell.com



On 2010-07-12 5.56 PM, László Sándor wrote:
Thank you, Michael!

I acknowledge this behavior, but I find it strange (= not very
intuitive for most uses of this) and also somewhat contrary to the
manual's explanation (below). So I need to add another arbitrary level
just to be able to introduce the level indicators I originally
intended to? How clumsy...

Thanks again,

Laszlo



Selecting levels


     You can select a range of levels -- a range of virtual variables -- by
     using the i(numlist). operator.


          Examples          Description
          --------------------------------------------------------------------
          i2.cat            a single indicator for cat==2


          2.cat             same as i2.cat


          i(2 3 4).cat      three indicators, cat==2, cat==3, and cat==4;
                              same as i2.cat i3.cat i4.cat


          i(2/4).cat        same as i(2 3 4).cat


          2.cat#1.sex       a single indicator that is 1 when cat==2 and
                              sex==1, and is 0 otherwise


          i2.cat#i1.sex     same as 2.cat#1.sex
          --------------------------------------------------------------------

On Tue, Jul 13, 2010 at 2:21 AM, Michael N. Mitchell
<Michael.Norman.Mitchell@gmail.com>  wrote:
Greetings

  The trick is that the -i.- is creating a reference group, so in your
example group 3 is the reference group. Here are some examples using the
-auto- dataset.

. sysuse auto
(1978 Automobile Data)

* group 1 is the reference, groups 2-5 are shown
. summ   i.rep78

    Variable |       Obs        Mean    Std. Dev.       Min        Max
-------------+--------------------------------------------------------
       rep78 |
          2  |        69     .115942    .3225009          0          1
          3  |        69    .4347826    .4993602          0          1
          4  |        69    .2608696    .4423259          0          1
          5  |        69    .1594203    .3687494          0          1

* group 1 is the reference, groups 2-5 are shown
. summ   i(1/5).rep78

    Variable |       Obs        Mean    Std. Dev.       Min        Max
-------------+--------------------------------------------------------
       rep78 |
          2  |        69     .115942    .3225009          0          1
          3  |        69    .4347826    .4993602          0          1
          4  |        69    .2608696    .4423259          0          1
          5  |        69    .1594203    .3687494          0          1

* group 1 is the reference, groups 2-4 are shown
. summ   i(1/4).rep78

    Variable |       Obs        Mean    Std. Dev.       Min        Max
-------------+--------------------------------------------------------
       rep78 |
          2  |        69     .115942    .3225009          0          1
          3  |        69    .4347826    .4993602          0          1
          4  |        69    .2608696    .4423259          0          1

* group 1 is the reference, groups 2-3 are shown
. summ   i(1/3).rep78

    Variable |       Obs        Mean    Std. Dev.       Min        Max
-------------+--------------------------------------------------------
       rep78 |
          2  |        69     .115942    .3225009          0          1
          3  |        69    .4347826    .4993602          0          1

* group 1 is the reference, group 2 is shown
. summ   i(1/2).rep78

    Variable |       Obs        Mean    Std. Dev.       Min        Max
-------------+--------------------------------------------------------
     2.rep78 |        69     .115942    .3225009          0          1


I hope that helps.

Michael N. Mitchell
Data Management Using Stata      - http://www.stata.com/bookstore/dmus.html
A Visual Guide to Stata Graphics - http://www.stata.com/bookstore/vgsg.html
Stata tidbit of the week         - http://www.MichaelNormanMitchell.com



On 2010-07-12 4.21 PM, László Sándor wrote:

Hi all,
I think I have a straightforward question in the subject line.

I use the up-to-date Stata on a mac. The factor variable handling does
not work as promised in the manual (cf. -h fvvarlist-). See the most
striking, simple example below. Or am I missing the obvious?

Thank you,

Laszlo



. sum i(3 4).role

     Variable |       Obs        Mean    Std. Dev.       Min        Max
-------------+--------------------------------------------------------
       4.role |       747    .2503347     .433496          0          1

. sum i(3/4).role

     Variable |       Obs        Mean    Std. Dev.       Min        Max
-------------+--------------------------------------------------------
       4.role |       747    .2503347     .433496          0          1
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