[ncl-talk] Problem using ttest function

[Anne]

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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