The author has identified the following significant results. The screening procedure resulted in a substantial decrease in the official LACIE winter acreage estimate, bringing the two estimates into better agreement with corresponding USDA/ESCS estimates

Chhikara, R. S.

English

NASA Technical Reports Server

  
 
 
N O T I C E 
 
THIS DOCUMENT HAS BEEN REPRODUCED FROM 
MICROFICHE. ALTHOUGH IT IS RECOGNIZED THAT 
CERTAIN PORTIONS ARE ILLEGIBLE, IT IS BEING RELEASED 
IN THE INTEREST OF MAKING AVAILABLE AS MUCH 
INFORMATION AS POSSIBLE 
https://ntrs.nasa.gov/search.jsp?R=19800005340 2020-03-21T20:30:36+00:00Z
August 1979 LEC-12123
	
Lockheed	 ^-	 - a:,ax 	 ^ cRElectron i -3-14 -S411cs Tel. 713"°id 1. Houston. Texas 7705e	 16 O31,T
Company, Inc.
Ref: 644-1269
"Mtn Mlleble under NASA sponsorship
	
Contract NAS 9-15800
In the int^;^^t a! ear !f a ;^^ wi'c dis-	 Job Order: 74-715
semirlo n,7 of ra th Resov, ces S;m ►ay	
AOi i 1 •= 1979F:ojrvrd 4"01 ,421 on and wifuut 1101ity
to any use made thereof."
TECHNICAL MEMORANDUM
A REVISED SCREENING PROCEDURE FOR LARGE AREA CROP
INVENTORY EXPERIMENT (LACIE) PHASE III DATA
IN THE U.S. GREAT PLAINS
By
R. S. Chhlkara
(280-10016) LARGE AREA CHOP INVENTORY
	 N80-13595
EYPEEINENT (LACIE) . A REVISED SCREENING
PROCEDURE FOR LACIE PHASE 3 DATA IN THE US
GREAT PLAINS (Lockheed Electronics Co.) 9 p
	 Unclas
HC A02/n? A01
	 CSCL 02C G3/43 00016
1 Dom-
	
;,	 Approved By:
Carroll, Manager
Earth Observations
Systems Department
ca(
.MYi	
Oii.lYnti 1' 'Etr^A4, &1^+`>^ E^AC^
CONTENTS
$Wi on
1. INTRODUCTION. . . . . . . . . .
2. THE SCREENING PROCEDURE	 .
3. REVISED VERSUS OFFICIAL LACIE P
ACREAGE ESTIMATES . . . . . . .
4. CONCLUSION. . . . . . . . . . .
S. REFERENCES. . . . . . . . . . .
Page
1
1
RASE III
3
5
6
1. INTRODUCTION
The screening procedure implemented in Phase III of the Large Area Crop Inven-
tory Experiment (LACIE) for detecting Classification and Mensuration Subsystem
(CAMS) segment estimates which indicated a s+ n;iificant change from the histor-
ical wheat acreages of their corresponding -:,#dnties was based on the extreme
studentized deviate statistic. However, the di:^ribution of the statistic was
assumed normal, and the method of testing many outliers present in a data set
was not adequately developed at the time of its implementation. Since then,
these defects have been corrected, and a screening procedure that utilizes
appropriate statistical methodology has been implemented on the Crop Assess-
ment Subsystem (CAS) Development System.
2. THE SCREENING PROCEDURE
The basic approach to screening remains the same as that given in reference 1.
{	 The variable defined by the ratio r - y/X, where y is the avera ge of CAMS
wheat proportion estimates for sample segments in a county and X is the his-
torical wheat proportion for the county, provides an estimate of the change in
wheat acreage of a county in a current year from X. In reference 1, the ratio
r - y/X, where y is the CAMS wheat proportion estimate for a segment in the
county, was considered for an estimate of this change in wheat acreage; how-
ever, when a large within-county variance exists for the CAMS segment esti-
mates, as observed for certain counties in Colorado (ref. 2), r is likely to
be an unreliable estimate for the county, as a bias is possible. Conversely,
the average ratio r can be regarded as a better estimate, since the use of r
eliminates the bias in a county estimate that might be caused by deletion of
!	 a subset of segments. The logarithmic transformation is applied to the values
of r to maintain the normal approximation hold for the underlying distribution.
Muter and spring wheat regions are treated separately; each region is strat-
ified by the size of historical wheat acreage at the county level. The
winter wheat region is divided into the following four strata:
Sl
	(X: 0 < X 4 5)
S2 n (X: 5<X <15)
S3 - {X: 15-< X < 30)
S4 - {X: X > 301
For the spring wheat region, three strata are formed:
SC U M 0<X<5)
Si - {X: 5 < X < 25)
Si - M X > 25)
There is one less stratum in the spring wheat case mainly because its region is
smaller than the winter wheat region.
The statistical procedure for testing outliers is quite different from the
one used in reference 1. The critical values for the test of significance
do not correspond to the percentage points of the normal distribution;
instead, these values are developed by using the Monte Carlo technique for
the test statistic computed for the normal samples. Simulations are used
because the exact distribution of the test statistic cannot be obtained.
The significance test is developed to detect as many as 19 outliers in a
data set. Thus far in the statistical literature, detection has been
developed for a maximum of four outliers (ref. 3). This test procedure was
documented in reference 4.
The screening was applied to final (Phase III) CAMS segment estimates
obtained after thresholding from each of the above strata. Counties were
flagged whose z values (where z - log r) were declared outliers. Consequently,
2
1
a
` ^	 ,^,	 4r	 urn,	 x r	 r. #f^ 	 -: H°t r^-^^._ , 
.1^'	 f	
^^.#^ tS
CANS segment estimates for these counties were deleted from the CAS data baseq
and the counties were treated as "Group III" in the CAS aggregation.
The revised U.S. Great Plains (USGP) winter and spring wheat acreage estimates
by states are presented in table 1. Also given are the official LACIE Phase III
estimates. The numerical results show that there is a better agreement between
the revised estimates and the U.S. Department of Agriculture/Statistical
Reporting Service' (USDA/SRS) (end of season) estimates when compared to the
agreement between the official LACIE Phase III estimates and the USDA/ESCS
estimates.
3. REVISED VERSUS OFFICIAL LACIE PHASE III ACREAGE ESTIMATES
The revised and the official LACIE Phase III acreage estimates given in
table 1 were obtained on the CAS Development System using the LACIE
Phase III CAS data base with thresholding and screening applied to the final
CAMS segment estimates. Thresholding precedes screening and is the same in
both cases; thus, the difference between the two estimates is due only to
the use of different screening procedures. The official estimates (column 5)
correspond to the screening procedure employed previously in LACIE Phase III,
and the revised estimates (column 9) correspond to the updated procedure
discussed in the previous section. (There is a slight difference between
the estimates given in column 5 and the officially reported LACIE estimates
because of a difference in the number of significant digits to which the
CAMS estimates were carried in the two CAS systems — development and
operational.)
A state-by-state comparison between the two estimates shows that the revised
LACIE winter wheat acreage estimates are closer to their corresponding
USDA/ESCS estimates. The only exception is for the stets of Oklahoma, where
the difference between the two estimates was slightly larger. For the seven
3
E_
Now called the Economics, Statistics, and Cooperative Service (ESCS).
3
CD ^ N N ^
•" rr r
^ M fit O
N N N
N. A N N iA
O on
r-
...
c"
1
Ln01 P4% 01 d
M	 w
N^ M 1ff d^ N N
eh
• N O^ a N e
rf NNat M
cy
^ eh
w
	
^ ^	 ft
O GlH 
	 pp
	
2	 ^ d v N N N ca
	 ^"q ^ M N^
	 rN
w	 ^a
O	
G
W	 t^	 O^	 M 00 P'f
	
VILn qr O &n 
f L
440	 40
r 0
	
... d	 M^ eh 1f! .p pt	 M M Cal N O1 N
	 Ln	
a
N 41
	
tV
L	
M g	 ch	
u v
w •.^ yF c O tp cn	 .^ N. 10 1p $ C eh fh	 N0 0	 ,^ v O e•► eh f0 01 eh eq
	 L. a
	
4n	 06LLJ 49
ap
41
	
Cwt
	
co
L
	
F.0  
C^	 '
	
a	 a ^p
ul N d 01 N	 f0
Go	 ,—.	
•et N 
N r. a N
	
e•f N fn ew 
CO
w	 ..^
V 
x	 U	 eV eV N
	
N "" N fn 01
	
.
	
r-	 b e} N
	
N
 MM 01 N N 1p
	
N
C
	
p-	 C r
N 01 N M
	
r
O .r	 M 01	 M O O to m 01	 di 0.
O CO .-	 ^^pp ►^ m1p	]{	 Oko ch 1p N r O	 v ^+ cm f, .r ++
	
to	 ^ d	 N	 .^.	 N fit 01 co	 L
•-	 "^	 s
cs
Cli
w
0 E
.°	
o	 a	
o	 ^►°	 °	 0 o	 '
v	 'oo c^	 cl v 
V
^pp
	
qq	
w
a	
O 1-a- O C	 C L C z V1 t 0
4
42
z
t
W
u^
N
W
t 
yyCD,^
Ldp O
O
1N
1-
HW
a uj
a 
0^.
Ww
,n
f^
fL V
O
W QC
CL
W
iii
.-.
C MO w
y^ .x.. aL
N
r
w W N
^r-
states combined, the difference between the LACIE estimate and the USDA/ESCS
F-	 estimate was reduced from 640 000 square hectometers (1.58 million acres)
to 239 000 square hectometers (0.59 million acres) as a result of updating
the screening procedure.
In obtaining the revised estimate, 325 CAMS segment estimates were used
compared to 298 CAMS segment estimates used for the LACIE estimate at the
seven-state level, a deletion of 27 segments. This is an expected result
provided the assumption of a uniform change in county wheat acreages from 	 i
epic year to current year holds for counties in each stratum. The outlier
test procedure applied in reference 1 is conservative, as it tends to declare 	 f
false outliers more often than is allowed under the 5-percent level of
significance presently used. Although the revised screening flags counties
and results in deleting all segments in them, compared to flagging and
deleting individual segments, there should be no adverse effect. 	 I
No significant change is noticed in spring wheat estimates obtained using
different screening procedures. There is a difference of 110 121 square
hectometers (272 000 acres) between the revised and the official LACIE
estimates, with the largest change in South Dakota. The revised spring
wheat acreage estimate for South Dakota is below the ESCS estimate by 3.7 per-
cent; the previous estimate was 1608 percent below the ESCS estimate. There
is very little change in the LACIE estimates of the three other spring wheat
states of Minnesota, Montana, and North Dakota.
4. CONCLUSION
The revised screening procedure has a sound statistical basis and eliminates
the two major drawbacks of the reference 1 procedure. The revision resulted
in a substantial decrease in the official LACIE winter wheat acreage estimate
and some increase in the spring wheat acreage estimate, bringing the two
estimates into better agreement with corresponding USDA/ESCS estimates.
5
S. REFERENCES
1. A Segment Screening Procedure for U.S. Great Plains, an attachment to
letter from R. S. Chhikara to W. McAllum, dated Sept. 9, 1977.
2. Colorado Blind Sites (Action Item RTEB SF3-134). A memorandum from
Jon Erickson to the Transition Year Project Manager, dated June 15, 1978.
3. Rosner, Bernard: Percentage Points for the RST Many Outlier Procedure.
Technometrics, 19, Aug. 1977.
4. Chhikara, R. S.; and Feiveson, A. H.: A Statistical Test Procedure for
Detecting Multiple Outliers in a Data Set, JSC-14594, Technical Report,
LEC-12910, Nov. 1978.
6


Large Area Crop Inventory Experiment (LACIE). A revised screening procedure for LACIE phase 3 data in the US Great Plains

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server