Re: [ferret_users] correlation coefficient using variance.jnl

[Ms Sindu Raj Parampil <parampil@xxxxxxxxxxxxxxxxxx>]

Hello Ansley,
Thank you for the suggestion. I listed the different variables used in 
calculation of correlation from both cases. 
Variance.jnl calculates CORREL= COVAR / (P_VAR_MASK*Q_VAR_MASK)^.5

For case 1 (i.e., p = isoairs, q = isorama and correl = 0.2269),
COVAR = 0.4739, P_VAR_MASK = 1.506, Q_VAR_MASK = 2.897

For case 2 (i.e., p = isorama, q = isoairs and correl = undefined),
COVAR = 0.4551, P_VAR_MASK = 2.931, Q_VAR_MASK = 1.504

So case 2 does have values upto the penultimate step. Yet, 
its not calculating the correlation. When these values are 
manually plugged in the expression, 
CORREL= COVAR / (P_VAR_MASK*Q_VAR_MASK)^.5 
the CORREL = 0.2168. This is still different from CORREL case 1 (0.2269). 

Meanwhile a friend mentioned that the different grids of variables 
(isoairs, isorama) could be reasponsible for the different CORRELs. So I 
wrote the variables out into a text file and read them onto the same grid. 
This solved the problem and now both CORRELs match! 

thanks and regards
Sindu


On Thu, 17 Sep 2009, Ansley Manke wrote:

> Hi Sindu,
> Yes, the correlation should be the same when p and q are reversed, and a 
> simple test I just tried does result in the same correl regardless of 
> which variable is P and which is Q.  Something odd seems to be happening 
> with your data.
> 
> One thing to try is after the first command
> 
>    let p = isorama[k=1]; let q = isoairs[k=1]; go variance ; li correl
> 
> then do "cancel variable/all; cancel memory",  before
> 
>    let p = isoairs[k=1]; let q = isorama[k=1]; go variance ; li correl
> 
> which will make sure that the data is read again and everything is 
> reinitialized.  I don't really know that this will make any difference. 
> 
> If the result of the second definitions is still missing, to try to find 
> out what might be going on, you could look at the variables that are 
> defined by the variance.jnl script:
> 
> yes? go variance
> yes? go/help variance  ! lists the commands in the script
> 
> Then just list or plot the values of the different variables that are 
> defined as the script builds up the definition of correl. For instance
> 
> list p_ave, q_ave
> list p_dev, q_dev      ! or plot p_dev, q_dev
> 
> and so forth.  This may show that one variable has lots of missing data, 
> perhaps, or some other peculiar behavior that's causing this.
> 
> Please write back and let us know what you find out.
> 
> 
> Ansley
> 
> 
> Ms Sindu Raj Parampil wrote:
> > Hello,
> > I've encountered a problem while calculating correlation using the 
> > 'variance.jnl' between 2 variables: isorama and ISOAIRS.
> >
> > When I choose p = isorama & q = isoairs to run variance.jnl, the  
> > correlation coefficient is 0.2269. But when p and q are reversed
> > (i.e., p = isoairs & q = isorama ), the correlation is undefined. 
> >
> > Shouldn't the corr. coeffs be same irrespective of the order of the 
> > variables for simultanoeus correlation?
> >
> > Pls see below for the commands I used in both cases and the ferret 
> > calculations.
> > 1) let p = isorama[k=1]; let q = isoairs[k=1]; go variance ; li correl
> >
> >              VARIABLE : CORRELATION OF P AND Q
> >              FILENAME : RH_1s90e_03_04.nc
> >              LONGITUDE: 90E
> >              LATITUDE : 1.5S
> >              DEPTH (m): -3
> >              TIME     : 31-DEC-2002 12:00 to 08-JUN-2004 12:00
> >           0.2269
> >
> > 2) let p = isoairs[k=1]; let q = isorama[k=1]; go variance ; li correl
> >  
> >              VARIABLE : CORRELATION OF P AND Q
> >              FILENAME : RH_1s90e_03_04.nc
> >              LONGITUDE: 89.5E
> >              LATITUDE : 1.5S
> >              HEIGHT (millibar): 1000
> >              TIME     : 31-DEC-2002 12:00 to 08-JUN-2004 12:00
> >         ....
> >
> > I use FERRET v6.2 on OpenSuse 11.0. Any help is appreciated. 
> > Thank you
> > Sindu
> >
> >   
> 
> 

-- 
Sindu Raj Parampil
Ph.D Student
Centre for Atmospheric and Oceanic Sciences
Indian Institute of Science
Bangalore

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