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st: - uniform() - random variable not uniformly distributed overinterval


From   "Deborah Garvey" <DGarvey@scu.edu>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: - uniform() - random variable not uniformly distributed overinterval
Date   Wed, 09 Mar 2005 17:19:32 -0800

Dear listers:

I am generating a random number with the goal of (randomly) assigning
children who report multiple races to a single race category for
purposes of analysis.  I'd like to evenly distribute children across
single race categories.  However, when I use uniform() to generate a
random number, I don't get an even split:

. gen r = uniform() if k_race == 801
(363374 missing values generated)

. su r

    Variable |       Obs        Mean    Std. Dev.       Min        Max
-------------+--------------------------------------------------------
           r |      2309    .4995691    .2914078   .0007709   .9993796

. gen byte k_race2 = .
(365683 missing values generated)

. replace k_race2 = 100 if r <= 0.5
(1151 real changes made)

. replace k_race2 = 200 if r > 0.5 & r <= 1
k_race2 was byte now int
(1158 real changes made)

I am puzzled.  I would expect median = mean = 0.5 for a uniform number
defined over [0,1). This must be a simple, basic question, but neither
changing the - set seed -, changing the interval to [0,1], or changing
the observations selected seems to solve this problem.  A search of
on-line help, the list archives and the Stata FAQs didn't give me much
insight either.

Can you help me figure out what I'm doing wrong?

Thanks,

Deb Garvey

******************************
Deborah Garvey, Ph.D.
Department of Economics
Kenna Hall
Santa Clara University
Santa Clara, CA  95053
408/554-5580
408/554-2331 (FAX)
dgarvey@scu.edu    
http://lsb.scu.edu/~dgarvey
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