[SciPy-dev] Problem with F distribution, or with me?

Alan Jackson alan@ajackson....
Mon Aug 11 21:19:36 CDT 2008


I'm confused. I was working on the documentation for the F-distribution,
and I'm getting results from the example that I don't understand.

I set the numerator degrees of freedom to 1 ( 2 groups)
and the denominator degrees of freedom to 48 (25 members in each group)

If I look up in a table, F(p<.01) = 7.19. I checked on the website
http://davidmlane.com/hyperstat/F_table.html
with 1, 48 and F=7 and got P = 0.01099

But when I run f in numpy, I get no values larger than about 0.65

 s = np.random.f(1, 48, 1000000)
In [40]: max(s)
Out[40]: 0.649036568048
 
I would have expected to see about 1% of the values > 7.19. Am I missing
something stupid?

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