Re: cdfgam_p

Dear Dennis,

Thanks for the suggestion. It (cdfgam_p(x,shape,1/scale)) works fine
now. Mean and variance are not mention in the NCL-webpage. Therefore I
guess it will be easier for user if the function declare as
cdfgam_p(x,shape,1/scale) instead of cdfgam_p(x,shape,scale) in the
webpage or exceptions in other software (excel, openoffice) are
mentioned in the webpage.

Thanks again

Debasish

Quoting Dennis Shea <shea@ucar.edu>:

> I asked a statistician
>
> The problem comes from the convention in the parametrization.
>
> Some languages use as input for the two parameters that characterize a
> Gamma distribution a and b such that a/b is the mean of the
> distribution and a/(b2) is the variance. Some languages use a and b
> such that a*b is the mean and a*(b2) is the variance.
>
> Excel evidently uses the opposite of NCL. The difference would not
> appear in the first example since b=1 so whatever the convention is
> in the first example the Gamma has mean 3 and variance 3. In the
> second example depending on the convention it either has mean 2/43 or
> 2*43 and variance 2/432 or 2*432 which makes a big difference.
>
>
> I just checked in R and if I ask pgamma(50.29,2,43) I indeed get 1,
> while if I ask pgamma(50.29,2,1/43) I obtain 0.32... like Excel.
>
> So tell your user to make sure he enters the parameters such that they
> give him the needed mean and variance for the gamma distribution.
>
>
>
> debasish@gi.alaska.edu wrote:
>> Dear Dennis,
>>
>> Thanks for your response. I just used function "GAMMADIST" from
>> microsoft excel to calculate the cumulative distribution.
>>
>> For
>> GAMMADIST(6.29,3,1,TRUE) = 0.949787846 which is well agreed with NCL
>>
>> but for
>> GAMMADIST(50.29,2,43,TRUE) = 0.32633483 while ncl gives 1
>>
>> Actually I have a timeseries with values of the order of 50 and
>> shape=2 and scale =43. Using "GAMMADIST" in excel give me expected
>> cumulative distribution curve but NCL returns 1 in all cases.
>>
>> I am sending my excell sheet attached with this mail where x
>> mentioned in column B, shape in G, scale in H and cumulative
>> distribution function in column I. The pot is given in chart3.
>>
>> Thanks again for helping me
>>
>> Debasish
>>
>> Quoting Dennis Shea <shea@ucar.edu>:
>>
>>> I am not sure wher you go that he correct answer is 0.34
>>>
>>> Using the R-statistical package
>>>
>>>> pgamma(6.29,3,1)
>>> [1] 0.9497878
>>>
>>>> pgamma(50.29,2,43)
>>> [1] 1
>>>
>>> These agree with NCL.
>>>
>>> D
>>>
>>>
>>> debasish@gi.alaska.edu wrote:
>>>> Hello,
>>>>
>>>> I just like to know is there any error in the function "cdfgam_p"
>>>> It works fine for the example mentioned in the web page
>>>> i.e. p=cdfgam_p(6.29,3,1); p=0.95
>>>>
>>>> but for
>>>> p=cdfgam_p(50.29,2,43); p=1 instead of 0.34
>>>>
>>>> thanks
>>>>
>>>> Debasish
>>>>
>>>>
>>>>
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Received on Fri Feb 19 13:20:09 2010