From: heaven_bird334 <heaven_bird334_at_nyahnyahspammersnyahnyah>

Date: Mon Apr 09 2012 - 09:15:34 MDT

Date: Mon Apr 09 2012 - 09:15:34 MDT

Dear Dennis,

Thank you very much for all your quickly efficient help. Please take a rest.

Happy Easter to you!

Best Regards,

Li

At 2012-04-09 21:27:38,"Dennis Shea" <shea@ucar.edu> wrote:

*>Hi Li,
*

*>
*

*>(1) The population mean is the 'avg' , 'dim_avg', 'dim_avg_n'
*

*>(2) Population variance can be readily calculated from the
*

*> sample variance.
*

*>
*

*> N = num(.not.ismissing(f))
*

*> var1 = variance(f)*(N-1)/N
*

*>
*

*>(3) There are no plans to create population versions of the
*

*> assorted function you mention.
*

*>
*

*>(4) Could you please post to ncl-talk@ucar.edu ?
*

*> I spent 6+ hours answering emails sent directly to me.
*

*> I can not spend that much time any more.
*

*>
*

*>Cheers
*

*>D
*

*>
*

*>
*

*>On 4/8/12 8:47 PM, heaven_bird334 wrote:
*

*>> Dear Dennis,
*

*>>
*

*>> Thank you very much for your quickly efficient response on Easter holidays.
*

*>>
*

*>> Now, I have some further questions, which are my real concerns:
*

*>>
*

*>> 1) Does NCL have the functions to calculate population variance or
*

*>> population mean now? If not, will your team develop new ones or update
*

*>> the old ones in the future?
*

*>>
*

*>> 2) Some times, an correlation or auto-correlation function will cause a
*

*>> correlation coefficient bigger than 1. This kind of problem are usually
*

*>> due to using sample mean/variance (divides by [1/(N-1)]) instead of
*

*>> using population mea n/variance (divides by [1/N]) in calculation, or
*

*>> say sample correlation/covariance functions. I notice that NCL's
*

*>> correlation/covariance, auto-crrelation/auto-covariance functions, such
*

*>> as "esacr", "esacv", "esccr", "esccv", "escorc", "escovc" et al., are
*

*>> all sample ones. Does NCL have population correlation/covariance,
*

*>> auto-crrelation/auto-covariance functions? If not, will your team
*

*>> develop new ones or update the old ones in the future?
*

*>>
*

*>> Happy Easter to you!
*

*>>
*

*>> Beset Regards,
*

*>> Li
*

*>>
*

*>>
*

*>>
*

*>> At 2012-04-09 00:35:16,"Dennis Shea" <shea@ucar.edu> wrote:
*

*>>>All 'variance' functions compute the sample variance which is
*

*>>>an unbiased estimate.
*

*>>>
*

*>>> f = (/ 7, 9, -2, -8, 2/)
*

*>>>
*

*>>> favg = avg(f) ; average
*

*>>> fdev = f-favg ; deviations
*

*>>> fdev2 = fdev^2 ; deviations squared
*

*>>> nf = dimsizes(f) ; N
*

*>>>
*

*>>> pvar = sum(fdev2)/nf ; population variance
*

*>>> svar = sum(fdev2)/(nf-1) ; sample variance
*

*>>>
*

*>>> var1 = variance(f)
*

*>>> var2 = dim_variance(f)
*

*>>> var3 = dim_variance_n(f,0)
*

*>>>
*

*>>> print("pvar="+pvar+" svar="+svar+" var1="+var1+" var2="+var2+"
*

*>>>var3="+var3)
*

*>>>===
*

*>>>output: pvar=37.84 svar=47.3 var1=47.3 var2=47.3 var3=47.3
*

*>>>
*

*>>>The quote below "... population variance" is incorrect. It will
*

*>>>be changed to 'sample variance'. Thank you.
*

*>>>
*

*>>>
*

*>>>
*

*>>>On 4/7/12 11:34 PM, heaven_bird334 wrote:
*

*>>>> Hi,
*

*>>>> Did anybody know the value calculated by the function "dim_variance"
*

*>>>> /"dim_variance_n" is "population variance" or "sample variance"?
*

*>>>>
*

*>>>> I am a little puzzle after I read the following sentence on
*

*>>>> http://www.ncl.ucar.edu/Document/Functions/Built-in/dim_variance.shtml:
*

*>>>> "Technically, this function calculates an estimate of the population
*

*>>>> variance. This means that it divides by [1/(N-1)] where N is the total
*

*>>>> number of non-missing values."
*

*>>>>
*

*>>>> As I know (please see
*

*>>>> http://en.wikipedia.org/wiki/Variance#Population_variance_and_sample_variance),
*

*>>>> the population mean o r population variance means it divides by [1/N],
*

*>>>> not by [1/(N-1)]. Does the NCL web documents get a written mistake there?
*

*>>>>
*

*>>>> Any suggestion or information is greatly appreciated.
*

*>>>>
*

*>>>>
*

*>>>> Best Regards,
*

*>>>> Li
*

*>>>>
*

*>>>>
*

*>>>>
*

*>>>>
*

*>>>>
*

*>>>>
*

*>>>>
*

*>>>>
*

*>>>>
*

*>>>> _______________________________________________
*

*>>>> ncl-talk mailing list
*

*>>>> List instructions, subscriber options, unsubscribe:
*

*>>>> http://mailman.ucar.edu/mailman/listinfo/ncl-talk
*

*>>
*

*>>
*

*>>
*

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Received on Mon Apr 9 09:15:58 2012

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