Re: RE: st: RE: Random assignment

[Richard Williams <Richard.A.Williams.5@nd.edu>]

At 05:42 PM 1/1/2005 +0800, =?GB2312?B?yNm58w==?= wrote:
> i am new to stata,so i can't understand some stat functions.
> can anyone explains what the following function means--y=uniform()<=0.6.
> thank you!
I was curious too, and your question inspired me to (hopefully) figure it 
out.  I figured it created a 0-1 dichotomy, and here is how I think it 
works.  From the online docs:

"uniform() returns uniformly distributed pseudo-random numbers on the 
interval [0,1).  This function takes no arguments, but the parentheses must 
be included."

So, if you just said, gen y=uniform() you would get a variable with values 
ranging from 0 to 1, with a uniform distribution, i.e. the mean would be 
around .5 but values distant from the mean would be just as likely to occur 
as values close to the mean.  (You can try this and see for yourself.)

Now, given that this var has a uniform distribution, about 60% of the 
values will be <=.6, while about 40% of the time the values will be greater 
than .6

So, when you give the command, gen y=uniform()<=0.6, about 60% of the time 
the statement will be true, returning a value of 1; about 40% of the time 
it will be false, returning a value of 0.

So, the result is that y is a 0-1 dichotomy with a mean of around .6 (i.e. 
about 60% of the cases are coded 1, the rest are 0).  It won't be exactly 
.6 because of random variability; the bigger your sample size is, the more 
likely it is to be about .6.

In the original problem the author had 50 missing cases that he wanted to 
replace with 0s and 1s, proportionate to the numbers of 0s and 1s that were 
present in the nonmissing data.  So if, say, 60% of the nonmissing cases 
were 1s, about 60% of the missing cases could be randomly assigned a value 
of 1 while the rest were assigned 0s with the above command.

This is a nice way to create a dichotomy, but it might be nicer still to 
have a command something like gen y = dichot(.6), which would mean create a 
0-1 dichotomy where 60% of the cases are 1s.  I suppose it would be easy 
enough to create such a command yourself if you had frequent need for it.

-------------------------------------------
Richard Williams, Associate Professor
OFFICE: (574)631-6668, (574)631-6463
FAX:    (574)288-4373
HOME:   (574)289-5227
EMAIL:  Richard.A.Williams.5@ND.Edu
WWW (personal):    http://www.nd.edu/~rwilliam
WWW (department):    http://www.nd.edu/~soc

*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/