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Re: st: losing observations while correcting for selection bias using selmlog


From   "Ramani Gunatilaka" <ramani.gunatilaka@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: losing observations while correcting for selection bias using selmlog
Date   Sun, 20 Apr 2008 10:44:07 +0600

Dear Joao Lima,
Thank you for your response. Sample size by select_m depvar category
is as follows, there are no missing values:
tabulate selmcat

categorical
variable
showing
employment
sector
choice       Freq.	Percent	Cum.
		
1       1,884	6.36	6.36
2      12,238	41.34	47.70
3       3,723	12.58	60.28
4       5,043	17.03	77.31
5       6,716	22.69	100.00
		
Total      29,604	100.00

. sum w3

Variable        Obs	Mean	Std. Dev.	Min	Max
				
w3       3723	4.013316	1.095727	-6.050407	6.69982

Public sector wage variable W3 has values for all 3723 public employee
observations and missing values for the observations relating to other
employment categories.

. sum $basefactors1

Variable        Obs	Mean	Std. Dev.	Min	Max
				
male      29604	.6367721	.4809379	0	1
age      29604	38.82702	13.52627	11	93
agesqd      29604	1690.491	1133.976	121	8649
SL_Tamil      29604	.042832	.2024817	0	1
I_Tamil      29604	.056614	.2311074	0	1
				
Moor      29604	.0472571	.2121919	0	1
other      29604	.0040873	.0638022	0	1
jsecondary      29604	.2399676	.4270706	0	1
ssecondary      29604	.5070599	.4999586	0	1
tertiary      29604	.0330699	.178822	0	1
				
managerial      29604	.0839751	.2773552	0	1
professional      29604.0482029	.2141984	0	1
technician      29604	.0463789	.2103078	0	1
clerical      29604	.0352317	.1843681	0	1
service      29604	.0671869	.2503496	0	1
				
agricultural      29604	.2110526	.4080625	0	1
elementary      29604	.2822929	.4501227	0	1
manuf      29604	.2341913	.4234995	0	1
services      29604	.3759627	.4843786	0	1
rural      29604	.8120524	.3906769	0	1
				
estates      29604	.0733685	.2607448	0	1
CP      29604	.1397784	.3467628	0	1
SP      29604	.1732874	.3785019	0	1
NWP      29604	.1327523	.3393126	0	1
NCP      29604	.0744494	.2625053	0	1
				
Uva      29604	.0816444	.2738268	0	1
Sab      29604	.118261	.3229224	0	1

I don't have any missing values for $basefactors1 variables - almost
all are 0/1 dummies.

. sum $selfactors

Variable	Obs	Mean	Std. Dev.	Min	Max
					
nempld	29604	2.234901	1.141517	0	9
nempld_m	29604	5.670382	5.652145	0	54
children	29604	.6210985	.8557174	0	6
mother	29604	.140758	.3477776	0	1
hh_head	29604	.4264626	.494571	0	1
					
married	29604	.6940616	.4608115	0	1
malemarried	29604	.4621673	.4985751	0	1

There are no missing values for the demographic variables used for the
selection equation.

Thank you for your help.
Best,
Ramani


On 19/04/2008, Joao Ricardo F. Lima <jricardofl@gmail.com> wrote:
> Ramani,
>
> please give us more information. How many observations do you have?
> Are there missing values to the independent variables? Could you send
>
> sum w3 $basefactors1 selmcat $selfactors
>
> I used selmlog and with missing values, the program drop that observations.
>
> Best,
>
> Joao Lima
>
> 2008/4/19, Ramani Gunatilaka <ramani.gunatilaka@gmail.com>:
> > Dear Statalist,
> > I am using selmlog to estimate wage functions. The full sample is of
> > labour market participants.
> > For the multinomial logit part, I have five categories - 1.
> > unemployed, 2. contributing family workers, own account workers,
> > employers, 3. public employees 4. private formal employees 5. informal
> > employees.
> > I have wage data only for categories 3, 4 and 5. I want to estimate
> > wage functions for each of these. As suggested by Rafa de Hoyes in an
> > old Statapost (12 April 2006), I defined separate wage variables for
> > each of these wage categories.
> > w3 is the wage data for public employees. There are 3723 of them.
> >
> > Now when I run the following model for the public sector, the eventual
> > wage equation ends up with only about 400 observations.
> >
> > selmlog w3 $basefactors1, select(selmcat=$basefactors1 $selfactors) showmlogit
> >
> > Can somebody please tell me why this is so?
> > BTW, I intend bootstrapping to fix the standard errors but will embark
> > on that time-consuming exercise once I sort out this problem.
> > Thank you in advance,
> > Ramani
> > *
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> >
>
>
> --
> -------------------------------
> Joao Ricardo Lima
> Professor
> UFPB-CCA-DCFS
> +553138923914
> -------------------------------
> *
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>
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