Re: equiv_sample_size

From: Dennis Shea (shea AT XXXXXX)
Date: Thu Aug 21 2003 - 10:45:11 MDT

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    Hello,

    I see the problem. For the surface air temperature variable
    data were available at all grid points. For SST there are some
    grid points [ ~33% :-) ] that are all missing because they are
    over land. The underlying fortran routine correctly set the internally
    calculated lag-1 autocorrelation as missing (_FillValue). At this point
    it should have returned with the equivalent sample size set to
    _FillValue also. Unfortunaltely, it did not.

    A fix will be applied in the near future.

    Regards
    Dennis Shea

    >Hi. I had a question about equivalent sample size and ftest.
    >I created variables sst1(lat,lon,time) and sst2(lat,lon,time),
    >and applied a low-pass filter to them. Then I calculated their
    >variances. For land pts. I use the FillValue of 1e+35.
    >Now, I want to determine if their variances are significantly
    >different using the ftest. I included my approach below.
    >However, the equiv_sample_size command leads to this error:
    >
    > ==>SLATEC/NCL: DGAMMA:
    > ANSWER LT HALF PRECISION BECAUSE X TOO NEAR NEGATIVE INTEGER: NERR=

    [SNIP]
    >
    >Previously, I did the same thing for surface air temperature without any
    errors.
    >Any idea what is wrong here? Thanks.
    >Mike
    >
    >**********************
    >
    > xEqv = equiv_
    >sample_size (sst1, 0.1,0)
    > yEqv = equiv_sample_size (sst2, 0.1,0)
    > gwt = latGauWgt (128, "lat", "gaussian weights", "dimension_less ")
    > xN = wgt_areaave (xEqv, gwt, 1., 0)
    > yN = wgt_areaave (yEqv, gwt, 1., 0)
    >
    > alpha=new((/128,128/),float)
    > alpha = 100.*(1. - ftest(var1,xN,var2,yN,0))
    > alpha!0="lat"
    > alpha!1="lon"
    > alpha&lat=lat
    > alpha&lon=lon
    > alpha@_FillValue=1e+35
    > alpha=mask(alpha,oro,0)
    >
    >
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