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st: Inconsistent Result of generating variables


From   "Jitian Sheu" <jtsheu@bu.edu>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: Inconsistent Result of generating variables
Date   Fri, 9 Apr 2004 17:12:17 -0400

Hello, listers:

I am not sure whether it is a bug in Stata?

Before going on, I will like to post my code first, then that would be
easier to explain:

----Stata code begins here---
clear

set obs 10

set seed 1204

gen x1=uniform()

gen emusq1=uniform()

gen emu1=uniform()

gen x2=x1^2

local b01=0.02

local b11=0.3

local b21=-0.25

gen tmptau1=emusq1+`b01'^2+(`b11'^2)*(x1^2)+(`b21'^2)*(x1^4)-2*`b01'*emu1-2*
/*

        */
`b11'*emu1*x1-2*emu1*`b21'*x1^2+2*`b01'*`b11'*x1+2*`b01'*`b21'*x1^2+2*`b11'*
`b21'*x1^3

gen
tmptau1_alt=emusq1-2*emu1*(`b01'+`b11'*x1+`b21'*x2)+(`b01'+`b11'*x1+`b21'*x2
)^2

list tmptau1 tmptau1_alt

tab tmptau1

tab tmptau1_alt

---- Stata code ends here-----

Basically, what I am doing is to use two formula to generate variable.

The first variable, tmptau1, use the extended formula to generate variable

The second one, tmptau1_alt, use a non-extended formula to generate variable

Hypothetically, because these two formula are exactly the same, Stata should
return two equal variables. (i.e. tmptau1== tmptau1_alt)

However, when I list and tabulate these two variables.....(as list)

We can find that tmptau1 is not "EXACTLY" the same with tmptau1_alt.

Does anyone know the reason why?

Thanks



Jitian



. list tmptau1*

+---------------------+
tmptau1   tmptau~t
---------------------
1.  .8692592   .8692592
2.   .199401   .2181771
3.  .0975708   .2075188
4.  .1246213   .1309747
5.  .6214758   .6345458
---------------------
6.  .4019333   .4080512
7.  .2606208   .2966675
8.  .2964657   .3682937
9.  .0604077   .0604092
10.  .9241236   .9241688
+---------------------+

. tab tmptau1

tmptau1       Freq. Percent Cum.

.0604077           1 10.00 10.00
.0975708           1 10.00 20.00
.1246213           1 10.00 30.00
.199401           1 10.00 40.00
.2606208           1 10.00 50.00
.2964657           1 10.00 60.00
.4019333           1 10.00 70.00
.6214758           1 10.00 80.00
.8692592           1 10.00 90.00
.9241236           1 10.00 100.00

Total          10 100.00

. tab tmptau1_alt

tmptau1_alt       Freq. Percent Cum.

.0604092           1 10.00 10.00
.1309747           1 10.00 20.00
.2075188           1 10.00 30.00
.2181771           1 10.00 40.00
.2966675           1 10.00 50.00
.3682937           1 10.00 60.00
.4080512           1 10.00 70.00
.6345458           1 10.00 80.00
.8692592           1 10.00 90.00
.9241688           1 10.00 100.00

Total          10 100.00





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