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Re: st: RE: Generating skewed distributions on closed intervals


From   Reza C Daniels <rdaniels@commerce.uct.ac.za>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: RE: Generating skewed distributions on closed intervals
Date   Thu, 29 Sep 2005 10:41:02 +0200

Hi again, again!

I've found a solution to the uniform distribution in the -egen var=seq() from() to()- command.

Is it not simpler just to try and transform this into the three appropriate normal and skewed distributions than to use the -betaden- set of commands? If so, how? If not, I revert to below.

Thanks again,
Reza




Reza C Daniels wrote:

Hi again,

I'm not sure I'm getting the intuition behind the code of the beta density functions -betaden- and -nbetaden-. My reading suggests using -betaden- for the symmetric ~ about 25, and -nbetaden- for the skewed ~s about 22.5 & 27.5.

However, when I plug in the numbers I get a single result. Clearly I'm doing something very wrong. Does this mean I need to calculate a & b & lambda (shape paramaters in betaden commands) first somehow?

Thanks,
Reza


Maarten Buis wrote:


you can have a look at the beta distribution
a normal distribution will never stay within an interval (except [minus infinity, plus infinity])


-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu]On Behalf Of Reza C Daniels
Sent: donderdag 29 september 2005 9:25
To: statalist@hsphsun2.harvard.edu
Subject: st: Generating skewed distributions on closed intervals

Dear Statalisters

I have a categorical variable for agegroup in 10 year bands (e.g. 20-30
years old). I would like to convert the categorical age variable to a
continuous variable by imposing various distributions on the range of
each interval. I then want to conduct sensitivity analysis to my
distributional assumptions.

For example: let a = the lower limit and b = upper limit for each age
group (e.g. a= 20 years old, b= 30 years old). Keeping the [20,30]
example, the four distributions I want to examine are:

1) Uniformly distributed over [20,30].
2) Normally distributed on the closed interval [20,30], with mode at 25.
3) Positively skewed on the closed interval [20,30], with mode at 22.5.
4) Negatively skewed on the closed interval [20,30], with mode at 27.5.

I have tried various commands (including -drawnorm-), but am unable to
control my variance to ensure the tails are bounded by [20,30] in the
example above (generically, the interval [a,b]).

Any suggestions on the code for all four distributions above would be
very much appreciated.

With thanks,
Reza
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