st: Fuction Fden Help

[Bryan Griffin <bwgriffin@georgiasouthern.edu>]

Hi --

I would like to create a plot showing density of F distribution for various
degrees of freedom, but I'm having difficulty getting this to work. I was
not able to find an example for the F distribution in the archives or on
the net, but I found this example which works well for the normal
distribution:

graph twoway function normden(x, 0, 1) , range(-5 5)

So using that as an example, I tried a number of variations of this

twoway function Fden(3,20,1)

No matter what numbers I place within the Fden() function the returned
graph is a flat line.

I also tried specifying a range, like this

twoway function Fden(3,20,1), range(.1 100)

but, again, no matter which numbers I try the graph returns a flat line.

Any suggestions?

Thanks,
Bryan

P.S. Here's the Function help information:

Title

  [D] functions -- Functions


Probability density function

  Fden(n1,n2,f)
     Domain n1:    1e-323 to 8e+307 (may be nonintegral)
     Domain n2:    1e-323 to 8e+307 (may be nonintegral)
     Domain f:     -8e+307 to 8e+307
                   Interesting domain is f > 0
     Range:        0 to 8e+307
     Description:  returns the probability density function for the F
distribution with n1 numerator and n2 denominator degrees of freedom.
returns 0 if f < 0.
*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/