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OCR for page 42
4
Categorical Eligibility
of Infants and Children
The first step both in eligibility determination and in estimating eligi-
bility is to determine if an individual meets the categorical eligibility re-
quirements, that is, whether the applicant is an infant, child, pregnant
woman, breastSeeding postpartum woman, or nonbreastSeeding postpar-
tum woman. As mentioned earlier, the current method for estimating eligi-
bility for WIC uses the March Demographic Supplement of the Current
Population Survey (CPS) to obtain a count of infants ages O through 12
months and the number of children ages 1 through 4 years. Since the num-
bers of pregnant and postpartum women cannot be directly observed in the
CPS, estimates of the number of women in these categories are based on
estimates of the number of infants in the CPS. Thus, the accuracy of the
estimate of the number of income-eligible infants is especially important in
. · .
the estimation process.
The panel's Phase I report found an undercount of infants in the CPS
(National Research Council, 2001: Table 3-11. This undercount ranged
between 1.0 and 4.1 percent in 1992-2000, and averaged over 2 percent a
year. The CPS underestimated the number of children in 1992 and 1993
but overestimated that number in the years 1994-2000.
In this chapter, we consider why the CPS estimates undercount infants
and outline a procedure that USDA can use to adjust the CPS estimates to
more accurately estimate the numbers of infants and children. The chapter
concludes with a general discussion of how the Survey of Income and Pro-
42
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CATEGORICAL ELIGIBILITY OFINFANTS AND CHILDREN
43
gram Participation (SIPP), an alternative data set for estimating WIC eligi-
bility, compares to the CPS in estimating the number of infants and chil-
dren.
EXPLANATION FOR THE UNDERCOUNT OF INFANTS
In its Phase I report, the panel hypothesized that the underestimation
of infants is at least partially due to the age groupings for black and other
race individuals used in the development of posts/ratification weights (Na-
tional Research Council, 20011. CPS-based estimates of the numbers of
persons less than 1 year old and 1 through 4 years are determined largely by
the sampling weights. The last step in the development of CPS weights,
sometimes referred to as a posts/ratification or population control adjust-
ment, compares CPS estimates with the available Census Bureau popula-
tion projections by age, gender, and race. These projections are derived
through a month-by-month adjustment of decennial census counts that
adds births, subtracts deaths (both births and deaths statistics come from
vital records), accounts for net migration, and corrects for the decennial
census undercount. The adjustment procedure is completed using total
population projections available at the time of final CPS weight construc-
tion. ~
CPS posts/ratification adjustments are done in age, gender, and race
subgroups of sufficient sample size to yield adjustment factors that are
stable. For infants and children, adjustments are made for each gender by
single year of age for white respondents, two-year age groups for black
respondents (0-1, 2-3, and 4-5 years) and a five-year age grouping (0-5
years) for other race respondents. These adjustment groups provide the
larger sample sizes needed to obtain more stable values of the population
control adjustments by gender.
Because the numbers of black and other race infants are not required
The projection counts may, at the time of the CPS adjustment, not be based on the
estimated number of births or deaths, since vital statistics used to provide final projections
are not available until two to three years after the projection year. As vital statistics data
become available, the Census Bureau releases new projections on a continuing basis for each
year and month throughout the course of a decade. Thus, estimates of population counts
derived from CPS weights do not agree with more recent Census Bureau population projec-
tions for March of a given year.
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44 ESTI~TING ELIGIBILI>~D PAR TICIPATION FOR THE ~CPROG~
to match control totals for single-year age intervals, the estimated number
of infants overall from the CPS does not match the number of infants in
the control totals. The underestimation of infants may also be partly due to
reporting error in the CPS respondents may tend to push up the age of
infants to 1 year when they are really only 10 months old. Or it may be the
result of other unknown factors.
The panel examined the nature of the differences between CPS esti-
mates and Census Bureau projections for single years of age for each of the
three racial groups used in the adjustment process. Table 4-1 presents a
comparison ofthe March CPS estimates and census projections ofthe num-
ber of children in single-year age intervals for each racial group for the year
2000. The pattern of the ratios, repeated across years, clearly shows that
during the decade, CPS estimates substantially underestimate infants for
black and other races and modestly underestimate the number of infants
TABLE 4-1 Census Projections and CPS Estimates by Single-Year Age
Groups, March 2000
White Black Other Total
Age 0
Census projection 3,103,504 623,345 235,485 3,962,334
CPS estimate 3,102,955 560,460 204,477 3,867,892
Ratio of projection to estimate 1.00 1.11 1.15 1.02
Age 1
Census projection 3,092,302 604,020 228,588 3,924,910
CPS estimate 3,091,779 596,446 272,324 3,960,549
Ratio of projection to estimate 1.00 1.01 0.84 0.99
Age 2
Census projection 3,059,341 593,856 231,388 3,884,585
CPS estimate 3,058,792 611,779 213,566 3,884,137
Ratio of projection to estimate 1.00 0.97 1.08 1.00
Age 3
Census projection 3,065,235 587,374 230,898 3,883,507
CPS estimate 3,064,706 584,498 228,661 3,877,865
Ratio of projection to estimate 1.00 1.01 1.01 1.00
Age 4
Census projection 3,121,016 608,019 230,192 3,959,227
CPS estimate 3,120,429 664,301 232,269 4,016,999
Ratio of projection to estimate 1.00 0.92 0.99 0.99
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CATEGORICAL ELIGIBILITY OFINFANTS AND CHILDREN
TABLE 4-2 Five-Year Accumulations of Census Projections and CPS
Estimates by Single-Year Age Groups, March 1996-2000
45
White
Black Other Total
Age 0
Census projection 15,508,050 3,060,394 1,141,043 19,709,487
CPS estimate 15,416,027 2,851,665 1,019,426 19,287,118
Ratio of projection to estimate 1.01 1.07 1.12 1.02
Age 1
Census projection 15,394,415 3,008,550 1,110,768 19,513,733
CPS estimate 15,407,985 3,182,528 1,198,862 19,789,375
Ratio of projection to estimate 1.00 0.95 0.93 0.99
Age 2
Census projection 15,434,140 3,045,150 1,117,163 19,596,453
CPS estimate 15,463,718 2,979,156 1,198,652 19,562,526
Ratio of projection to estimate 1.00 0.95 1.00 1.00
Age 3
Census projection 15,533,367 3,098,387 1,119,274 19,751,028
CPS estimate 15,533,536 3,221,080 1,127,924 19,882,540
Ratio of projection to estimate 1.00 0.96 0.99 0.99
Age 4
Census projection 15,873,802 3,260,672 1,123,277 20,257,751
CPS estimate 15,875,210 3,201,070 1,147,431 20,223,711
Ratio of projection to estimate 1.00 1.02 0.98 1.00
across all races. For example, the CPS estimate of black infants is 11 per-
cent below the census projection. The CPS estimates of white children of
different age groups match the control totals very closely. For children ages
1 through 4 years who are black or of other races, the CPS estimates are
sometimes too large and sometimes too small relative to control totals, but
they closely match control totals for some age groups.
Table 4-2 presents the ratios for a five-year accumulation of census
projections and CPS estimates.2 The ratios are more stable estimates of the
relationship between census projections and CPS estimates for the period
1996-2000 because they are based on five years of data. These ratios indi-
2The five-year interval was chosen to allow accumulation of sufficient CPS sample in
each survey year of the "other race" groups.
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46 ESTI~TING ELIGIBILI>~D PAR TICIPATION FOR THE ~CPROG~
care what appears to be a consistent underestimate of the number of non-
white infants in the CPS. The patterns of the ratios across single years of
age and races, as well as the total across races, are similar to the ratios ob-
served for the year 2000 comparison in Table 4-2.
ADJUSTMENT PROCEDURE TO IMPROVE THE ACCURACY
OF THE COUNTS OF INFANTS AND CHILDREN
The panel considered the size of the underestimation of infants sub-
stantial enough to suggest a procedure to adjust CPS estimates as a correc-
tion. The five-year accumulated ratios of the census projections to the CPS
estimates like those presented in Table 4-2 could be used to adjust the
weight value for an individual in a given age and racial group in the corre-
sponding cell in the table. For example, to estimate the number of infants
in 2001, the cumulative ratios from 1996-2000 would be used to change
the weights for the 2001 estimates. Use of these ratios would slightly in-
crease the weight given to a white infant (multiply the CPS individual
weight by 1.01), increase the weight given to a black infant (multiply the
CPS individual weight by 1.07), and increase the weight given to an infant
whose race falls into the "other" category (multiply the CPS individual
weight by 1.12). A similar adjustment would be applied to each age and
race group. Furthermore, for the sake of the panel's exercise, males and
females were combined. The CPS weights are developed separately by gen-
der, so for a complete adjustment, separate adjustments should be made for
males and females.
Table 4-3 shows the results of this adjustment procedure using the
1996-2000 ratios for 2001 CPS estimates by age. A shift in estimates from
older ages to infants is indicated by the percentage relative change for each
age. For example, the adjustment results in a 2.1 percent increase in the
total number of infants in 2001 and a 1.3 percent decrease in the total
number of 1-year-olds.
There are several ways to use such accumulated ratios to adjust the
CPS weights. In the example given in Table 4-3, we used accumulated
ratios from five past years to estimate the next year's population (e.g.,1996-
2000 accumulated ratios were used to adjust 2001 estimates). On one hand,
to the extent that the ratios of accumulated data reflect stable trends over
time (that is, relatively unchanging from year to year), the accumulated
ratios from the 1996-2000 data could be made for several subsequent years,
such as 2002, 2003, 2004, and 2005. On the other hand, if five-year accu-
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CATEGORICAL ELIGIBILITY OFINFANTS AND CHILDREN
TABLE 4-3 Adjustment of 2001 CPS Estimated Population by Race
Using 1996-2000 Accumulated Census Projection to CPS Estimate
Ratios
47
White
Black
Other
Total
Age 0
Ratio of projection to estimate 1.006 1.073 1.119 1.022
Revised2001 CPS estimate 3,137,907 581,358 223,417 3,942,682
% relative change 2.13
Age 1
Ratio of projection to estimate 0.999 0.945 0.927 0.986
Revised2001 CPS estimate 3,094,949 657,197 223,060 3,975,206
% relative change -1.31
Age 2
Ratio of projection to estimate 0.998 0.945 0.998 1.002
Revised 2001 CPS estimate 3,093,102 615,377 262,134 3,970,613
% relative change 0.17
Age 3
Ratio of projection to estimate 1.000 0.962 0.992 0.993
Revised2001 CPS estimate 3,058,363 588,310 237,622 3,884,295
% relative change -0.64
Age 4
Ratio of projection to estimate 1.000 1.019 0.979 1.002
Revised 2001 CPS estimate 3,103,575 679,129 235,810 4,018,514
% relative change 0.17
The revised 2001 CPS estimates are made by multiplying the ratio of the 2001 popula-
tion estimates by the five-year accumulated adjustment ratios by age and race.
mutated ratios can be computed for each year (for example, 1997-2001,
1998-2002, and so on) and used instead of the 1996-2000 ratios, changes
in the trend of the ratios could be partially accounted for in the estimates.
For example, to adjust the 2003 CPS estimates of infants and children,
ratios accumulated over 1998,1999,2000,2001, and 2002 could be used.
The panel did not explore how stable the adjustments are over time to see if
one of these two methods is preferable. But such an activity should be
conducted before a specific procedure is chosen.
The panel does not give specific advice about how the adjustments are
created (e.g., whether the five-year accumulated ratios in Table 4-2 could
continue to be used in future years or if new five-year accumulated ratios
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48 ESTI~TING ELIGIBILI>~D PAR TICIPATION FOR THE ~CPROG~
should be estimated every year). The five-year adjustments derived and
presented in Table 4-2 should serve as a model for the general derivation of
the adjustment factors. The multiple years of data used will help stabilize
adjustments but still reflect changes in trends over time.
RECOMMENDATION: To accurately estimate the number of in-
fants and children using the CPS, USDA should apply five-year accu-
mulated ratio adjustment factors to individual CPS weights using a
procedure similar to the one outlined above. The adjustment factors
should be calculated separately by single-year age intervals for each of
the CPS control total race and gender groups.
SIPP-BASED ESTIMATES OF THE NUMBERS
OF INFANTS AND CHILDREN
SIPP could be used to produce estimates ofthe numbers of infants and
children who are eligible for WIC. SIPP, like the CPS, uses census popula-
tion projections by age, race, and gender to construct posts/ratification ad-
justments. The SIPP sample sizes are not large enough to support stable
estimates of population controls for the "other race" category. Thus, SIPP
population control adjustments are developed for each gender by single-
year age intervals for nonblacks and for two-year age intervals for blacks
(0-1, 2-3, and 4-51.
The panel compared SIPP estimates of the population from December
1997 with census projections for December 1997. The comparison shows
that the SIPP slightly overestimates the number of infants compared with
the census by 0.6 percent and overestimates the number of children by 4.1
percent. SIPP estimates of women of childbearing ages (15-45) for De-
cember 1997 overestimated the number of women relative to the census by
1.3 percent.
This comparison included only one month's estimates from SIPP. It is
difficult to draw conclusions about the accuracy of the counts of infants in
SIPP. Because the number of black infants is not required to match totals
for single-year age groups, it is likely that the count of black infants would
be underestimated relative to infants in the white category. In SIPP, people
of other races are included in the white race category. In the CPS, infants
reported as "other race" were underestimated more than white and black
infants, relative to control totals. Not having separate controls for this group
and including them in the nonblack category would reduce the problem of
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CATEGORICAL ELIGIBILITY OFINFANTS AND CHILDREN
49
underestimation of infants. However, the smaller SIPP sample sizes would
result, all else equal, in less stability of estimates of single-year age groups
for nonblacks and less stability in estimates of two-year age intervals for
blacks. The SIPP estimates of the number of income-eligible infants are not
used to estimate the number of pregnant and postpartum women as those
from the CPS are. However, the presence of an infant in the household is
used to infer that a woman was pregnant or that a woman is currently
postpartum. If infants are undercounted, then estimates of the number of
postpartum women will be undercounted as well. Whether or not this trans-
lates into an undercount of pregnant women depends on the reason why
the infant was not counted. If the household was missed entirely, then
information on the mother will not be available. If the mother of the infant
rounds the infant's age up to 1 year, the mother's pregnancy status can still
be inferred, but the timing to which pregnancy is attributed will be off by a
number of months.
SUMMARY
This chapter examines the undercount of infants and overcount of
children in the CPS relative to population control totals. The undercount
is especially problematic for blacks and for members of other nonwhite
racial groups. It appears to be at least partially due to the age groupings for
black and other race individuals when the CPS posts/ratification weights
are constructed. The panel recommends a procedure to adjust the CPS
estimates of the numbers of infants and children to correct for the
undercount. We did not examine whether such a procedure is needed to
adjust SIPP-based estimates of these groups. If SIPP is used to estimate
eligibility, further exploration of the accuracy of estimates of infants and
the implications of any accuracies on the estimates of pregnant and post-
partum women may be warranted.
Representative terms from entire chapter:
cps estimate
