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Re: st: xi3 / nlogit


From   "Michael N. Mitchell" <Michael.Norman.Mitchell@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: xi3 / nlogit
Date   Sat, 04 Dec 2010 18:33:50 -0800

Dear Nils

Unfortunately, I am out of my element with respect to the kind of model that you are fitting, but I hope that someone else might be able to offer some ideas about this.

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 2010-12-04 8.58 AM, Nils Wlömert wrote:
Many thanks Michael, the definition of the effect coded variables works great now.
However, I think there is another issue related to variable coding in the model (empty
cells that are coded as zeros).

I estimated the asclogit choice model (6 attributes with 4 levels each + no-chocie base
option) in Stata with linear coding of all attributes and the results are identical to the
results obtained for the 1 segment solution in Latent Gold Choice which als uses
McFadden's choice model:

Attributes
p_ppd (price I)
-0,4507
p_fl (price II)
-0,8754
p_drm
-0,1731
fl_drm
-0,5922
ad
-0,2764
cat
0,2817
nobuy
-1,6554

When it comes to effect coding, there are some difficulties in Stata. I first define the
effect coded variables as you suggested (price attributes remain linear coded):
. xi3 e.p_drm e.fl_drm e.ad e.cat e.nobuy
e.p_drm _Ip_drm_0-4 (naturally coded; _Ip_drm_0 omitted)
e.fl_drm _Ifl_drm_0-4 (naturally coded; _Ifl_drm_0 omitted)
e.ad _Iad_0-4 (naturally coded; _Iad_0 omitted)
e.cat _Icat_0-4 (naturally coded; _Icat_0 omitted)
e.nobuy _Inobuy_0-1 (naturally coded; _Inobuy_0 omitted)

When trying to estimate asclogit, some levels are dropped because of collinearity and the
model does not coverge.
asclogit decision p_ppd p_fl _Ip_drm_1 _Ip_drm_2 _Ip_drm_3 _Ip_drm_4 _Ifl_drm_1 _Ifl_drm_2
_Ifl_drm_3 _Ifl_drm_4 _Iad_1 _Iad_2 _Iad_3 _Iad_4 _Icat_1 _Icat_2 _Icat_3 _Icat_4
_Inobuy_1, noconst case(case) alternatives(alternative)
note: _Icat_4 dropped because of collinearity
note: _Iad_4 dropped because of collinearity
note: model has collinear variables; convergence may not be achieved

This is somewhat surprising as the model is estimated without any difficulties in Latent
Gold:

Attributes:
p_ppd (price I - linear)
-0,3540
p_fl (price II - linear)
-1,1689
p_drm (effect)
1 0,4179
2 -0,2083
3 0,2317
4 -0,4412
fl_drm (effect)
1 0,5906
2 0,3027
3 -0,2918
4 -0,6015
ad (effect)
1 0,4328
2 0,1001
3 -0,0839
4 -0,4489
cat (effect)
1 -0,5570
2 -0,0606
3 0,2858
4 0,3318
nobuy (effect)
0 0,4390
1 -0,4390

The same thing happens using nlogit:
nlogit decision p_ppd p_fl _Ip_drm_1 _Ip_drm_2 _Ip_drm_3 _Ip_drm_4 _Ifl_drm_1 _Ifl_drm_2
_Ifl_drm_3 _Ifl_drm_4 _Iad_1 _Iad_2 _Iad_3 _Iad_4 _Icat_1 _Icat_2 _Icat_3 _Icat_4
_Inobuy_1 || type: || alternative:, noconst case(case)
note: _Iad_2 dropped because of collinearity
note: _Icat_2 dropped because of collinearity
note: the model specified for level 2 has collinear variables; convergence may not be
achieved

I assume that this difference is related to the coding of the effect coded attributes as
the model is specified identically in Stata and Latent Gold otherwise.

Note that the model includes alternative specific attributes (e.g., price I & II) which
means that some attributes are NOT included in all alternatives (i.e., there are empty
cells in the data set if an attribute is not included in an alternative). Also, all cells
of the attribute levels are 'empty' in the lines representing the 'no-choice' alternative
. Latent Gold treats these cells as 'empty' and returns the part-worths for each level of
the effect coded attributes without omitting a level (see above). In Stata, these 'empty'
cells are replaced by zeros and the 0-levels are omitted when the model is estimated (see
above). I would be grateful if you could give some advice on how to define the effect
coded variables correctly. Especially with regard to the empty cells that are coded as
zeros and the handling of collinerity.

Thanks & best,

Nils



Am 04.12.2010 um 01:31 schrieb Michael Mitchell:

Dear Nils

When I wrote -xi3-, I don't think these models existed (or if they
did, I did not have them in mind). So, unfortunately, -xi3- does not
immediately work with programs like -xtmixed-, or -nlogit-, because it
gets confused by the pipes -||-. However, you can still use -xi3- if
you do it in a two step process. For example, here is using -xi3- in a
one step process to do a regression

. sysuse auto
(1978 Automobile Data)

. xi3: regress price e.rep78 g.foreign
e.rep78 _Irep78_1-5 (naturally coded; _Irep78_1 omitted)
g.foreign _Iforeign_0-1 (naturally coded; _Iforeign_0 omitted)

Source | SS df MS Number of obs = 69
-------------+------------------------------ F( 5, 63) = 0.19
Model | 8372481.37 5 1674496.27 Prob > F = 0.9670
Residual | 568424478 63 9022610.75 R-squared = 0.0145
-------------+------------------------------ Adj R-squared = -0.0637
Total | 576796959 68 8482308.22 Root MSE = 3003.8

------------------------------------------------------------------------------
price | Coef. Std. Err. t P>|t| [95% Conf. Interval]
-------------+----------------------------------------------------------------
_Irep78_2 | 188.879 1024.352 0.18 0.854 -1858.124 2235.882
_Irep78_3 | 646.8116 710.873 0.91 0.366 -773.7548 2067.378
_Irep78_4 | 274.3754 799.3802 0.34 0.733 -1323.059 1871.809
_Irep78_5 | 104.1799 1036.513 0.10 0.920 -1967.126 2175.486
_Iforeign_1 | 36.7572 1010.484 0.04 0.971 -1982.533 2056.048
_cons | 5797.125 581.597 9.97 0.000 4634.896 6959.353
------------------------------------------------------------------------------

Instead, we can first use -xi3- to create the coded variables....

. xi3 e.rep78 g.foreign
e.rep78 _Irep78_1-5 (naturally coded; _Irep78_1 omitted)
g.foreign _Iforeign_0-1 (naturally coded; _Iforeign_0 omitted)

And then we can include the coded variables into the model, as shown below.

. regress price _Irep78_2 _Irep78_3 _Irep78_4 _Irep78_5 _Iforeign_1

Source | SS df MS Number of obs = 69
-------------+------------------------------ F( 5, 63) = 0.19
Model | 8372481.37 5 1674496.27 Prob > F = 0.9670
Residual | 568424478 63 9022610.75 R-squared = 0.0145
-------------+------------------------------ Adj R-squared = -0.0637
Total | 576796959 68 8482308.22 Root MSE = 3003.8

------------------------------------------------------------------------------
price | Coef. Std. Err. t P>|t| [95% Conf. Interval]
-------------+----------------------------------------------------------------
_Irep78_2 | 188.879 1024.352 0.18 0.854 -1858.124 2235.882
_Irep78_3 | 646.8116 710.873 0.91 0.366 -773.7548 2067.378
_Irep78_4 | 274.3754 799.3802 0.34 0.733 -1323.059 1871.809
_Irep78_5 | 104.1799 1036.513 0.10 0.920 -1967.126 2175.486
_Iforeign_1 | 36.7572 1010.484 0.04 0.971 -1982.533 2056.048
_cons | 5797.125 581.597 9.97 0.000 4634.896 6959.353
------------------------------------------------------------------------------

I know it is a kludge, but I hope it works for you.

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 Fri, Dec 3, 2010 at 4:07 PM, Nils Wlömert <nils.wloemert@gmx.net> wrote:
Dear listers,

I would like to use effect-coding (via xi3) with nlogit:

xi3: nlogit decision p_ppd p_fl e.p_drm e.fl_drm e.ad e.cat e.nobuy || type:
|| alternative:, noconst case(case_1)

However, I get the following error:

"|" invalid name
r(198);

Does xi3 work with nested models at all?

Estimation works fine without effect-coding via xi3.

Many thanks!

Nils
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