[Numpy-discussion] Computing Simple Statistics When Only they Frequency Distribution is Known

Wayne Watson sierra_mtnview@sbcglobal....
Fri Nov 27 23:25:30 CST 2009


I actually wrote my own several days ago. When I began getting myself 
more familiar with numpy, I was hoping there would be an easy to use 
version in it for this frequency approach. If not, then I'll just stick 
with what I have. It seems something like this should be common.

A simple way to do it with the present capabilities would be to "unwind" 
the frequencies,  For example, given [2,1,3] for some corresponding set 
of x, say, [1,2,3], produce[1, 1, 2, 3, 3, 3]. I have no idea if numpy 
does anything like that, but, if so, the typical mean, std, ... could be 
used. In my case, it's sort of pointless. It would produce an array of 
307,200 items for 256 x (0,1,2,...,255), and just slow down the 
computations "unwinding" it in software. The sub-processor hardware 
already produced the 256 frequencies.

Basically, this amounts to having a pdf, and values of x. 
Mathematically, the statistics are produced directly from it.

josef.pktd@gmail.com wrote:
> On Fri, Nov 27, 2009 at 9:47 PM, Wayne Watson
> <sierra_mtnview@sbcglobal.net> wrote:
>   
>> How do I compute avg, std dev, min, max and other simple stats if I only
>> know the frequency distribution?
>>     
>
> If you are willing to assign to all observations in a bin the value at
> the bin midpoint, then you could do it with weights in the statistics
> calculations. However, numpy.average is, I think, the only statistic
> that takes weights. min max are independent of weight, but std and var
> need to be calculated indirectly.
>
> If you need more stats with weights, then the attachment in
> http://projects.scipy.org/scipy/ticket/604  is a good start.
>
> Josef
>
>
>   
>> --
>>           Wayne Watson (Watson Adventures, Prop., Nevada City, CA)
>>
>>             (121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
>>              Obz Site:  39° 15' 7" N, 121° 2' 32" W, 2700 feet
>>
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-- 
           Wayne Watson (Watson Adventures, Prop., Nevada City, CA)

             (121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
              Obz Site:  39° 15' 7" N, 121° 2' 32" W, 2700 feet  
                
                   350 350 350 350 350 350 350 350 350 350
                     Make the number famous. See 350.org
            The major event has passed, but keep the number alive.
 
                    Web Page: <www.speckledwithstars.net/>



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