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# Re: st: Difference operator will not calculate the correct difference

 From Dani Tilley To statalist@hsphsun2.harvard.edu Subject Re: st: Difference operator will not calculate the correct difference Date Sun, 8 Aug 2010 23:04:13 -0700 (PDT)

The problem is with what you expect it to be. The second difference is not the
difference between x and L2.x; it's the difference in the first difference of x.
So, d2.age=(age - l1.age) - (l1.age - l2.age).

----- Original Message ----
From: Duha Altindag <daltin2@tigers.lsu.edu>
To: statalist@hsphsun2.harvard.edu
Sent: Mon, August 9, 2010 12:48:52 AM
Subject: st: Difference operator will not calculate the correct difference

Greetings,
I thought the difference operator, D, worked like the following:
dn.x = x(t)-x(t-n)
n is an integer.
However, Stata is getting me something different, I couldn't figure out what.
Please, run the following to replicate the problem:

********begin*********
clear
use http://www.stata-press.com/data/r11/nlswork.dta
xtset  idcode year
g d2_age=age-l2.age
list  idcode year age l2.age d2_age d2.age ,noobs
*********end***********

And compare d2_age and D2.age columns.

------------------------------------------+
|                        L2.            D2.|
| idcode   year   age   age   d2_age   age |
|------------------------------------------|
|      1     70    18     .        .     . |
|      1     71    19     .        .     . |
|      1     72    20    18        2     0 |
|      1     73    21    19        2     0 |
|      1     75    23    21        2     . |
|------------------------------------------|
|      1     77    25    23        2     . |
|      1     78    26     .        .     . |
|      1     80    28    26        2     . |
|      1     83    31     .        .     . |
|      1     85    33    31        2     . |
|------------------------------------------|
|      1     87    35    33        2     . |
|      1     88    37     .        .     . |
|      2     71    19     .        .     . |
|      2     72    20     .        .     . |
|      2     73    21    19        2     0 |
|------------------------------------------|

Am I missing something?
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