[ncl-talk] question on dim_avg_n and dim_stddev_n

[Andrew Kren - NOAA Affiliate]

Dear ncl-talk,

I am computing a paired t test between control experiments sensitivity
experiments to look at hurricane track, wind, and intensity errors. I
compute the standard deviation of the differences and the avg of the
differences, as well as a 95% confidence interval based on the differences.

Attached is my code, see lines starting on 1127. Is this the correct way to
compute the standard deviation and average of the differences between
experiments? My data is a function of (storm case, forecast cycle, forecast
hour). Reason I ask is that when I do this procedure, the average
differences I get using this method as a function of forecast hour (which
is what I want) is this:

Variable: track_error_diff_avg (subsection)

Type: float

Total Size: 116 bytes

            29 values

Number of Dimensions: 1

Dimensions and sizes: [29]

Coordinates:

Number Of Attributes: 1

  _FillValue : 9.96921e+36

(0) 3.574069

(1) 5.169381

(2) 4.654985

(3) 1.50052

(4) 3.559848

(5) 3.355001

(6) 4.764093

(7) 8.347384

(8) 7.4192

(9) 8.69336

(10) 12.72247

(11) 17.61452

(12) 17.19171

(13) 11.51769

(14) 9.379377

(15) 11.44438

(16) 7.419726

(17) -3.023807

(18) -18.86928

(19) -3.98752

(20) -4.023374

(21) -14.1253

(22) -7.246421

(23) -10.78448

(24) -23.74559

(25) -40.31917

(26) -51.49041

(27) -34.30735

(28) -9.808663


However, when I compute the raw difference from the average track fields,
i.e. line 1154, I get these values:


Variable: track_error_diff_avg (subsection)

Type: float

Total Size: 116 bytes

            29 values

Number of Dimensions: 1

Dimensions and sizes: [29]

Coordinates:

Number Of Attributes: 1

  _FillValue : 9.96921e+36

(0) 3.574068

(1) 5.16938

(2) 4.654984

(3) 1.500519

(4) 3.559845

(5) 3.355003

(6) 4.764091

(7) 8.654808

(8) 8.30986

(9) 6.454712

(10) 10.45447

(11) 17.66672

(12) 20.24161

(13) 14.71471

(14) 13.84648

(15) 17.98428

(16) 9.601624

(17) -0.5063171

(18) -14.034

(19) -3.797974

(20) -0.2883911

(21) -11.9198

(22) -9.015442

(23) -6.522919

(24) -61.46265

(25) -74.77875

(26) -40.00131

(27) 52.12799

(28) 76.7471


where the values start to deviate after index 8. Is there something I am
doing wrong? I can't seem to find out why if so. Could it be there are
missing values for certain storms and cycles? Any help is much appreciated.

-- 
Andrew Kren
Research Scientist I, Global Observing Systems Analysis (GOSA) Group
NOAA ESRL Global Systems Division (Rm 3C515)
325 Broadway, Boulder, CO 80305
(303) 497-5418
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