# [ncl-talk] Problem using ttest function

Rashed Mahmood rashidcomsis at gmail.com
Mon Jan 21 17:07:49 MST 2019

```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:

>
> 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.
>>
>> 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.
>>>
>>> Anne
>>>
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>>
>>
>> --
>> 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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