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st: RE: Re: Re: xttrans


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: Re: Re: xttrans
Date   Tue, 20 Nov 2007 11:43:34 -0000

Look inside -xttrans- with 

. viewsource xttrans.ado

-xttrans- looks forward; your calculation 1)
looks backward. My guess is that you should
expect slightly different results given what
pairs are missing at the ends of each panel. 

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

A. Manzoni

1) The command:
sort pid month
bys pid: gen lagstatus=status[_n-1]
ta status lagstatus, row

2)the discrepancy:
Using xttrans
xttrans status, i(pid) t(month)
     
 RECODE of |
    newel2 |
(RECODE of |
     newel |  RECODE of newel2 (RECODE of newel (RECODE
(RECODE of |                of element))
 element)) |         E          S          N          U |     Total
-----------+--------------------------------------------+----------
         E |     99.21       0.05       0.62       0.12 |    100.00 
         S |      0.22      99.39       0.28       0.11 |    100.00 
         N |      2.04       0.06      97.84       0.06 |    100.00 
         U |      7.28       0.00       2.32      90.40 |    100.00 
-----------+--------------------------------------------+----------
     Total |     75.93       5.93      17.13       1.00 |    100.00 


Using
ta status lagstatus, row:
                            
 RECODE of |
    newel2 |
(RECODE of |
     newel |
(RECODE of |                    lagstatus
 element)) |         E          S          N          U |     Total
-----------+--------------------------------------------+----------
         E |    23,063          4        106         22 |    23,195 
           |     99.43       0.02       0.46       0.09 |    100.00 
-----------+--------------------------------------------+----------
         S |        12      1,797          3          0 |     1,812 
           |      0.66      99.17       0.17       0.00 |    100.00 
-----------+--------------------------------------------+----------
         N |       144          5      5,078          7 |     5,234 
           |      2.75       0.10      97.02       0.13 |    100.00 
-----------+--------------------------------------------+----------
         U |        27          2          3        273 |       305 
           |      8.85       0.66       0.98      89.51 |    100.00 
-----------+--------------------------------------------+----------
     Total |    23,246      1,808      5,190        302 |    30,546 
           |     76.10       5.92      16.99       0.99 |    100.00 

Subject: st: RE: xttrans

Show us 

1. The _exact_ code used to generate lagstatus. 

2. An example of discrepancies.  

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

Anna Manzoni

I am using the xttrans command. I have a longitudinal data file with
status in each month for each respondent (12 months for each
respondent). I do not understand why using

xttrans status, i(pid) t(month) 

doesn't give me exactly the same result as 

ta status lagstatus, row

where status is the var indicating the status in each month (t) and
lagstatus is status at t-1.
What is xttrans actually doing?

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