[SciPy-user] Is this a bug in stats.geom.pmf?

[Stefan van der Walt]

On Mon, Jun 25, 2007 at 02:59:09PM -0600, Fernando Perez wrote:
> just curious.  The stats.geom docstring says:
> 
>     Geometric distribution
> 
>     geom.pmf(k,p) = (1-p)**(k-1)*p
>     for k >= 1
> 
> But I see this:
> 
> In [10]: k,p = 2.0,0.5
> 
> In [11]: (1-p)**(k-1)*p
> Out[11]: 0.25
> 
> In [12]: stats.geom.pmf(k,p)
> Out[12]: array(0.125)
> 
> However:
> 
> In [13]: stats.geom.pmf(k-1,p)
> Out[13]: array(0.25)
> 
> 
> Is this an off-by-one bug, or am I misreading something here?

Yes, a mistake in geom._pmf that would never have shown up, since it
does not have a unit test.  I'll fix it, unless you beat me to it.

Cheers
Stéfan