[ncl-talk] Problem using ttest function

[Dennis Shea]

I have not looked at this in ge=reat detail.
However, *maybe*, the attached schemattic is the approach to use.

Good Luck

On Mon, Jan 21, 2019 at 5:08 PM Rashed Mahmood <rashidcomsis at gmail.com>
wrote:

> I think the line for prob should be as below (Note 1-ttest):
>
>  prob =  100.*(*1-*ttest(ave_s, VAR_s, xN, ave_c, VAR_c, yN, iflag,
> tval_opt))
>
> Not sure if this would solve the issue, but thought it might help?
>
> Cheers,
> Rashed
>
>
>
>
> On Mon, Jan 21, 2019 at 7:52 AM Anne <anne.seidenglanz at unive.it> wrote:
>
>> Hi Adam,
>>
>> thanks for your suggestion; indeed I suspected there could be issues with
>> the choice of xN and yN. In fact I have played with these numbers, but
>> unfortunately even setting them to 10 does not change much (see attached
>> plot). In any case I agree that 10 ensemble members is not a high number
>> for ensemble statistics. Do you think the strange probability pattern in
>> the plot could be due to this fact?
>>
>> thanks,
>> Anne
>>
>> On Fri, Jan 18, 2019 at 8:12 PM Adam Phillips <asphilli at ucar.edu> wrote:
>>
>>> Hi Anne,
>>> You are calling equiv_sample_size (which we usually recommend), and then
>>> your are calling wgt_areaave to arrive at one number:
>>>   xEqv = equiv_sample_size (jfm_s, sigr, 0)
>>>   yEqv = equiv_sample_size (jfm_c, sigr, 0)
>>>   xN   = wgt_areaave (xEqv, 1., 1., 0)    ; wgty could be gaussian
>>> weights
>>>   yN   = wgt_areaave (yEqv, 1., 1., 0)
>>>           prob =  100.*(ttest(ave_s, VAR_s, xN, ave_c, VAR_c, yN, iflag,
>>> tval_opt))
>>>
>>> To simplify things you could replace xN and yN with 10 (=the number of
>>> ensemble members, or sample size). But note: Depending on the field you are
>>> looking at 10 samples is not a large number, especially if there is high
>>> autocorrelation. This also might result in values coming out of
>>> equiv_sample_size = 1 or 0, which might cause issues in the ttest function.
>>>
>>> Hope that helps. If you have further questions let ncl-talk know.
>>> Adam
>>>
>>> On Thu, Jan 17, 2019 at 2:14 PM Anne <anne.seidenglanz at unive.it> wrote:
>>>
>>>> Hi everyone,
>>>>
>>>> I have some issues using the built-in 'ttest' function. When I plot the
>>>> 'raw' probability (so regardless of any significance level), I end up with
>>>> a rather strange plot, see attached.
>>>> I followed example 4 from the documentation at
>>>> https://www.ncl.ucar.edu/Document/Functions/Built-in/ttest.shtml, just
>>>> that instead of time I have ensemble member as the right-most dimension,
>>>> and hence compute mean and variance of the ensemble members. So my two
>>>> fields each have the following dimensions:
>>>> Dimensions and sizes: [Ens | 10] x [lat | 180] x [lon | 360]
>>>>
>>>> Fields for mean and variance look fine, but there is something wrong
>>>> with the probability plot. I attach also the script.
>>>>
>>>> Thanks for any advice,
>>>> Anne
>>>>
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>>>
>>>
>>> --
>>> Adam Phillips
>>> Associate Scientist,  Climate and Global Dynamics Laboratory, NCAR
>>> www.cgd.ucar.edu/staff/asphilli/   303-497-1726
>>>
>>> <http://www.cgd.ucar.edu/staff/asphilli>
>>>
>>
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