Re: [ferret_users] correlation coefficient using variance.jnl

[Ansley Manke <ansley.b.manke@xxxxxxxx>]

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
>
>