strange happenings!

[a.larkin@xxxxxxxx]

Hello,
please could someone help me on this, I  am very confused...(and probably begin daft)

I write the following script...

set data "xaafaa.pjhi10all.nc" - a file with 10 years of 12 months each
set data "xaafba.pjh9jan.nc" - a file with 1 month of data

I am basically practising commands to do a t-test. So, I want to use the simple formula, (uvals-uave)/usd  where uvals are my values to be tested, uave are my average values and usd is my standard deviation. (I am looking at zonal wind)

let uvals=U[d=xaafba.pjh9jan.nc,X=@ave]
let uave=U[d=xaafaa.pjhi10all.nc,X=@ave,T=@ave]
let usd=(U[d=xaafaa.pjhi10all.nc,X=@ave,T=@var])^0.5
;then to calculate my t-test values I do...
let utest=(uvals-uave)/usd

;then I contour utest
contour/x=1.88E:1.88W/y=88.75S:88.75N/z=100:997/t="15-jan-1979":"24-oct-1988" utest
;this gives me 2 error messages, one I understand and the other I do not.
1. ***NOTE: Ambiguous coordinates on T axis: (uvals-uave)/usd
2. ***ERROR: all data have the same value: data are all flagged as bad

I understand that by using both uvals and uave or uvar in the same formula, the time axis will be ambiguous but this is not usually a problem.
The strangest thing is that if I contour just any of (unorm-uvar), (unorm+uvar), (unorm*uvar) or (unorm/uvar) I get the same errors, but if I contour (uvar+unorm), (uvar-unorm), (uvar*unorm) or (uvar/unorm) I get no error messages.
Could someone please enlighten me as to what is happening.
Thanks
Alice Larkin