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Small-Area Estimates of School-Age Children in Poverty: Interim Report I: Evaluation of 1993 County Estimates for Title I Allocations
APPENDIX B
Comparison of Census and CPS Estimates of Poverty
There are many similarities and some differences between the CPS and the census systems for measuring poverty. They both base poverty estimates on the official definition of poverty for the nation, which compares the income of a family (or an unrelated individual) to a given poverty threshold, but they differ in the amount of data they collect, how they collect and process the data, and the frequency of data collection.
1990 CENSUS
A census of the U.S. population is conducted once every 10 years, and 1990 is the most recent one. In the 1990 census, income data—the basis for measuring poverty—were collected from a sample of 15 million households: a sample of about I in 6 households spread systematically across the country, except that very small counties and places (with estimated 1988 populations under 2,500) were sampled at a 1-in-2 rate, and very populous census tracts (or equivalent areas) were sampled at a 1-in-8 rate.
Data are collected in the census mainly by self-enumeration, whereby respondents fill out questionnaires received in the mail. Enumerators follow up those households that fail to return a questionnaire and collect the information through direct interviews. In 1990 approximately 74 percent of U.S. households returned their questionnaires with some or all of the requested information (Edmonston and Schultze, 1995:189). Data from the balance of the population were obtained by personal interviews. The follow-up enumerators are usually inexperienced temporary workers who are given very limited training.
The income data in the 1990 census are based on eight questions on various
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Small-Area Estimates of School-Age Children in Poverty: Interim Report I: Evaluation of 1993 County Estimates for Title I Allocations
components of income. (The census form also included a total income question, which was intended to permit respondents to enter a single amount if they could not provide amounts by source.) Nonresponse rates are higher for income than for most other items in the census. When household income information is missing, the Census Bureau uses statistical techniques to impute it on the basis of nearby households with similar characteristics. For the 1990 census, on average, 19 percent of aggregate household income was imputed (Edmonston and Schultze, 1995:387).
All censuses are subject to undercount—that is, failure to count everyone. There are no direct estimates of the undercount for poor children. For 1990, the net undercount was estimated at 1.8 percent for the total population, but there were substantial differences among population groups. For example, the net undercount was estimated at 5.7 percent for blacks and 1.3 percent for nonblacks. The net undercount also varied significantly by age: black girls and boys aged 5–9 were missed at a rate of 7.5 percent and 7.7 percent, respectively. Almost two-thirds of the estimated omitted population consisted of two age groups: children under age 10 and men aged 25–39 (Robinson et al., 1993:13). The undercount was also higher in large cities than in other areas, and it was disproportionately concentrated in the inner areas of those cities. These are also the areas where poverty is high. Thus, it seems likely that the undercount for poor children aged 5–17 is larger than the undercount of all children aged 5–17.
CURRENT POPULATION SURVEY
The CPS is a monthly labor force participation survey. For the period from 1990 to 1994, about 60,000 housing units were eligible for interview every month, and about 57,400 of them were found to be occupied by households eligible for interview.1 Of these 57,400 households, an interview was not obtained for various reasons for about 2,600 households—a noninterview rate of 4.5 percent.
Part of the CPS sample is changed each month: in the rotation plan, three-fourths of the sample is common from one month to the next, and one-half is common for the same month a year earlier. Each March, supplementary questions are asked about money income received the previous year. To obtain more reliable income data for the Hispanic-origin population, all November CPS households with one or more Hispanic persons are reinterviewed in March if they still include a Hispanic person. This procedure adds about 2,500 Hispanic households to the sample in March.
The CPS sample design, which is a multistage probability sample design, is revised about once every 10 years on the basis of the results of the latest census.
1
Starting in 1996, about 50,000 households nationwide (a sample of about 1 in 2,000 households) were eligible for interview every month—a reduction of about 17 percent from the early 1990s.
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Small-Area Estimates of School-Age Children in Poverty: Interim Report I: Evaluation of 1993 County Estimates for Title I Allocations
From 1986 to 1994, the CPS sample design included 729 sample areas consisting of about 1,300 counties. These areas were chosen on the basis of 1980 census data to represent all 3,141 counties (in 1990) and independent cities in the 50 states and the District of Columbia. A design based on the 1990 census was phased in between April 1994 and July 1995: it included 792 sample areas consisting of about 1,300 counties, chosen to represent all 3,143 counties (in 1994) and independent cities in the 50 states and the District of Columbia.
In general, larger states have larger CPS sample sizes. The largest states, however, have CPS sample sizes that are smaller than their proportionate share of the U.S. population, and the smallest states have proportionately larger sample sizes. For example, California, with 12.2 percent of the U.S. population, has 9.9 percent of the CPS sample; Wyoming, with 0.18 percent of the U.S. population, has 1.3 percent of the CPS sample. This sample design means that estimates of poverty rates in large states are generally more precise than those in smaller states. The largest states, however, have larger relative errors due to sampling variability than would be expected if the CPS sample were allocated to the states in proportion to their population; the reverse holds true for smaller states.
The sample is designed to meet specific reliability criteria for the nation, each of the 50 states and the District of Columbia, and the substate areas of New York City and the Los Angeles-Long Beach metropolitan area. The CPS is not designed for direct county estimates. More than one-half of U.S. counties do not have sample households in the survey.
The CPS is carried out by permanent, experienced, and well-trained interviewers, initially by personal direct interviews, with subsequent interviews by telephone. For the March Income Supplement, the CPS asks household respondents about their money income received during the previous year, using a detailed set of questions for identifying about 28 different sources. About 20 percent of aggregate household income is imputed (about the same percentage as in the census)—that is, the data are missing and therefore constructed from information from similar households (Citro and Kalton, 1993:Table 3-6).
Like other household surveys, the CPS exhibits population undercoverage at higher rates than the census itself. The coverage ratios for the CPS show the magnitude of the population undercoverage relative to the census. Coverage ratios are defined as the estimated survey population before ratio adjustment to census-based population controls divided by the census-based population controls. (Beginning with the March 1994 CPS, the population controls reflect an adjustment for the undercount in the census itself.) For March 1994, the ratio of the CPS estimated population to the population control total (all ages) was 92 percent; for the age group 0–14 and the age group 15–19 years, the ratios were 94 percent and 88 percent, respectively (Bureau of the Census, 1996:Table D-2).
CPS undercoverage is corrected by ratio adjustments to the survey weights that bring the CPS estimates of population in line with updated national population controls by age, race, sex, and Hispanic origin. However, the ratio adjust-
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Small-Area Estimates of School-Age Children in Poverty: Interim Report I: Evaluation of 1993 County Estimates for Title I Allocations
ments do not correct for other characteristics on which the undercovered population might be expected to differ from the covered population. For example, the ratio adjustments reweight equally the sample households within an age-race-sex-Hispanic origin category, when research suggests that it is likely that lower income households within a category are more poorly covered than higher income households.
DIFFERENCES BETWEEN CENSUS AND CPS DATA
In comparing the census and the CPS as data sources for estimates of income and poverty, one difference is the definition of the universe for which the numbers in poverty are estimated; see Table B-1. Residence rules differ for the two data sources: in the census, students attending colleges away from their parental homes are counted at their college location; in the CPS, they are usually counted at their parental home. Also, the census excludes all unrelated individuals under age 15 in households from the poverty universe; the CPS excludes only those unrelated individuals under age 15 in households who are not part of an unrelated subfamily. (Unrelated subfamilies are made up of people who are related to each other but not to the householder, such as the family of a resident employee.) For
TABLE B-1 Poverty Universes for the 1990 Census and the March 1990 CPS
Component
1990 Census
March 1990 CPS
Total resident U.S. population
248,709,873
250,180,762a
Population not covered in CPS
-3,998,221
-3,989,762
Institutionalized
-3,334,018
—b
Armed Forces in barracks
-589,700
—b
Unrelated individuals
under 15 in group quarters
-74,503
—b
Unrelated individuals under 15 in households
-780,235c
-199,000
College dormitory residents
-1,953,558
0d
Poverty universe
241,977,859
245,992,000
a Reflects 1980 census-based population estimates, which estimated a higher resident population in 1990 than the 1990 census.
b Intercensal estimates used to derive CPS population controls for survey weighting are not available by component. The sum of these three components was 3,989,762.
c The large difference between census and CPS estimates reflects the census practice of excluding children under age 15 in unrelated subfamilies from the poverty universe.
d The CPS includes household members away at school who are living in dormitories as family members and includes them in the poverty universe.
SOURCE: Data from Housing and Household Economic Statistics Division, Bureau of the Census.
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Small-Area Estimates of School-Age Children in Poverty: Interim Report I: Evaluation of 1993 County Estimates for Title I Allocations
estimates of related children in poverty (who are children in families but not children in unrelated subfamilies), the CPS and census poverty universes differ less than the CPS and census poverty universes for the total population. Essentially, the only difference is the treatment of college students in dormitories.
Overall imputation rates for income nonresponse are about the same in the census and the CPS, and the amount of income imputed is similar. Table B-2 presents aggregate income estimates for different income components from the two sources. The more detailed set of income questions in the CPS—28 compared with 8 in the census—and the direct interviewing methodology in the CPS would be expected to provide more comprehensive and accurate income data. It is believed that overreporting of some components of income, such as wages and salaries, occurs in the self-reported census data. (Another reason for the overestimate of wages and salaries in the census, compared with the independent benchmark, may be the editing procedures that were applied to responses to the total income question.)
The net effect of the differences between the CPS and the census in data collection, processing, and other aspects of the two systems is that there are dif-
TABLE B-2 Household Income by Type, 1989:1990 Census and March 1990 CPS
Aggregate Income (in $ billions)
Percentage of Benchmark
Source of Income
1990 Census
March 1990 CPS
Independent Benchmark
1990 Census
March 1990 CPS
Total money income
3,537.4
3,460.4
—
—
—
Amounts for which benchmarks can be computed
3,499.2
3,393.9
3,819.7
91.6
88.7
Wages and salaries
2,652.7
2,545.9
2,625.2
101.0
96.8
Nonfarm self-employment
218.6
207.1
290.0
75.4
71.4
Farm self-employment
20.3
18.6
49.9
40.7
37.2
Interest, dividends, and rent
258.8
247.7
471.5
54.9
52.5
Social Security and Railroad Retirement
188.2
201.4
207.9
90.5
96.9
Public assistance
28.3
25.9
32.8
86.3
79.0
Retirement, disability, and survivor income
132.3
147.3
142.4
92.9
103.4
Other income sources
38.2
66.5
—
—
—
NOTE: The independent benchmarks shown here for 1989 were extrapolated from 1990 independent estimates. For a detailed discussion of development of independent benchmarks, see Bureau of the Census (1993b:C1–C3).
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Small-Area Estimates of School-Age Children in Poverty: Interim Report I: Evaluation of 1993 County Estimates for Title I Allocations
ferences between them in estimates of income and poverty. Aggregate estimates of total income and income by type when compared with independent benchmarks differ between the CPS and census. For example, total income from the 1990 census (for income types for which independent estimates can be constructed) is 91.6 percent of the benchmark; the corresponding figure from the March 1990 CPS is 88.7 percent. For income from such sources as Social Security and Railroad Retirement and retirement, disability, and survivor income, the CPS is closer to the benchmark than the census.
Estimates of median household income in 1989 by state differ between the March 1990 CPS and the 1990 census by amounts that are statistically significant for 18 states; see Table B-3. For 15 of the 18 states, the census estimates are higher than the CPS estimates; for some states, the differences are as much as 10–15 percent. For three states, the census estimates of median household income are significantly lower than the CPS estimates, in the range of 6–8 percent. Estimates of poverty rates by state also differ between the CPS and the census: statistically significant differences are observed for seven states; for six of them, the census poverty rates are significantly higher; see Table B-4.
Estimates of poverty rates for related children aged 5–17 differ between the CPS and the census. The CPS estimate that 18 percent of all related children aged 5–17 were poor in 1989 (based on a 3-year average of data from the March 1989, 1990, and 1991 CPS) exceeds the census estimate by 1 percentage point (5.9% of the census estimate), a difference that is statistically significant at the 10 percent significance level; see Table B-5. (The CPS estimate of the number of poor school-age children also exceeds the census estimate by a statistically significant amount; see Table 2-4.)
A question is whether CPS and census estimates of poverty rates and numbers of poor related children aged 5–17 differ in terms of geographic distribution, perhaps because of differences in completeness of income reporting that reflect the income mix in different areas or other reasons. In preliminary research conducted by the panel, no statistically significant differences were found in the ratios of CPS to census estimates of the number of poor school-age children among various geographic groupings of counties and states. For example, when counties were grouped by size, there were no significant differences among the groups in their ratios of CPS to census estimates of the number of poor school-age children.
In summary, there are many factors reflecting differences between the census and CPS concepts and procedures that may account for variations in their estimates of poverty levels, rates, and distributions. It is important to keep these factors in mind, particularly when attempting to measure changes in poverty at specific levels of geography since 1990.
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Small-Area Estimates of School-Age Children in Poverty: Interim Report I: Evaluation of 1993 County Estimates for Title I Allocations
TABLE B-3 Median Household Income in 1989 by State: 1990 Census and March 1990 CPS
Income (in $)
1990 Census
March 1990 CPS
Percentage Difference: Census-CPS as Percent of CPS
State and National
Median
Standard Error
Median
Standard Error
U.S. Total
30,056
7
28,906
159
4.0*
Alabama
23,597
58
21,284
1,070
10.9*
Alaska
41,408
165
36,006
1,378
15.0*
Arizona
27,540
72
28,552
1,210
-3.5
Arkansas
21,147
50
21,433
915
-1.3
California
35,798
27
33,009
618
8.4*
Colorado
30,140
55
26,806
1,398
12.4*
Connecticut
41,721
77
42,321
1,592
-1.4
Delaware
34,875
151
32,068
1,133
8.8*
D.C.
30,727
156
26,752
1,015
14.9*
Florida
27,483
35
26,085
475
5.4*
Georgia
29,021
53
27,542
1,021
5.4
Hawaii
38,829
173
35,035
1,328
10.8*
Idaho
25,257
79
24,654
953
2.4
Illinois
32,252
32
31,300
623
3.0
Indiana
28,797
50
25,898
1,022
11.2*
Iowa
26,229
46
26,265
792
-0.1
Kansas
27,291
56
26,862
908
1.6
Kentucky
22,534
55
23,283
1,206
-3.2
Louisiana
21,949
49
22,861
1,857
-4.0
Maine
27,854
82
28,221
1,389
-1.3
Maryland
39,386
73
36,016
1,187
9.4*
Massachusetts
36,952
58
36,086
704
2.4
Michigan
31,020
32
30,775
790
0.8
Minnesota
30,909
37
30,185
1.278
2.4
Mississippi
20,136
59
19,917
947
1.1
Missouri
26,362
42
26,497
746
-0.5
Montana
22,988
98
23,692
1,311
-3.0
Nebraska
26,016
60
26,319
1,521
-1.2
Nevada
31,011
92
29,340
8,455
5.7*
New Hampshire
36,329
97
37,532
1,371
-3.2
New Jersey
40,927
51
39,120
948
4.6*
New Mexico
24,087
89
22,602
1,028
6.6
New York
32,965
41
31,496
453
4.7*
North Carolina
26,647
38
26,406
517
0.9
North Dakota
23,213
94
25,229
903
-8.0*
Ohio
28,706
37
29,021
655
-1.1
Oklahoma
23,577
57
23,667
1,236
-0.4
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Small-Area Estimates of School-Age Children in Poverty: Interim Report I: Evaluation of 1993 County Estimates for Title I Allocations
Income (in $)
1990 Census
March 1990 CPS
Percentage Difference: Census-CPS as Percent of CPS
State and National
Median
Standard Error
Median
Standard Error
Oregon
27,250
55
28,529
1,435
-4.5
Pennsylvania
29,069
32
28,690
684
1.3
Rhode Island
32,181
120
30,124
1,354
6.8
South Carolina
26,256
56
23,798
1,059
10.3*
South Dakota
22,503
89
24,108
999
-6.7*
Tennessee
24,807
50
22.611
1,305
9.7
Texas
27,016
27
25,886
559
4.4*
Utah
29,470
86
30.717
1,014
-4.1
Vermont
29,792
110
31,295
1,136
-4.8
Virginia
33,328
60
34,118
1,205
-2.3
Washington
31.183
44
31,961
1,472
-2.4
West Virginia
20,795
61
21,677
843
-4.1
Wisconsin
29,422
53
29,123
1,240
1.0
Wyoming
27,096
133
29,521
1,289
-8.2*
* Statistically significant difference from 0 at the 10 percent significance level.
SOURCE: Data from Bureau of the Census.
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Small-Area Estimates of School-Age Children in Poverty: Interim Report I: Evaluation of 1993 County Estimates for Title I Allocations
TABLE B-4 Poverty Rates for All Persons by State, 1989:1990 Census and March 1990 CPS
1990 Census
March 1990 CPS
State and National
Population (000s)
Total Poverty Rate
Standard Errora
Population (000s)
Total Poverty Rate
Standard Error
Difference Between Rates
U.S. Total
241,978
13.12
0.012
245,992
12.82
0.2
0.30
Alabama
3,946
18.34
0.096
4,074
18.90
2.0
-0.56
Alaska
532
9.00
0.140
488
10.45
1.5
-1.45
Arizona
3,584
15.74
0.086
3,556
14.12
1.9
1.63
Arkansas
2,292
19.07
0.116
2,419
18.27
2.0
0.80
California
29,003
12.51
0.027
29,346
12.85
0.7
-0.35
Colorado
3,213
11.68
0.080
3,258
12.06
1.8
-0.38
Connecticut
3,188
6.82
0.057
3,136
2.87
1.0
3.95*
Delaware
645
6.71
0.086
676
10.06
1.7
-1.35
D.C.
571
16.87
0.233
569
17.93
2.4
-1.06
Florida
12,641
12.69
0.042
12,762
12.46
0.9
0.23
Georgia
6,300
14.65
0.069
6,197
14.99
1.9
-0.34
Hawaii
1,071
8.25
0.119
1,036
11.23
1.8
-2.98*
Idaho
966
13.25
0.153
1,014
12.43
1.6
0.82
Illinois
11,144
11.91
0.033
11,559
12.72
0.9
-0.81
Indiana
5,372
10.68
0.051
5,453
13.74
1.9
-3.06
Iowa
2,677
11.48
0.074
2,835
10.26
1.5
1.22
Kansas
2,392
11.48
0.074
2,835
10.26
1.5
1.22
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Small-Area Estimates of School-Age Children in Poverty: Interim Report I: Evaluation of 1993 County Estimates for Title I Allocations
1990 Census
March 1990 CPS
State and National
Population (000s)
Total Poverty Rate
Standard Errora
Population (000s)
Total Poverty Rate
Standard Error
Difference Between Rates
Kentucky
3,582
19.03
0.093
3,576
16.11
2.0
2.92
Louisiana
4,101
23.58
0.108
4,080
23.28
2.3
0.29
Maine
1,190
10.80
0.108
1,233
10.38
1.6
0.72
Maryland
4,661
8.27
0.054
4,567
9.00
1.6
-0.73
Massachusetts
5,812
8.93
0.050
5,831
8.80
0.8
0.14
Michigan
9,077
13.12
0.040
9,297
13.21
0.9
-0.09
Minnesota
4,259
10.22
0.053
4,268
11.20
1.7
-0.98
Mississippi
2,503
25.21
0.147
2,574
21.99
2.0
3.22
Missouri
4,971
13.34
0.055
5,193
12.59
1.8
0.75
Montana
777
16.07
0.186
816
15.56
1.8
0.51
Nebraska
1,531
11.14
0.097
1,602
12.73
1.6
-1.59
Nevada
1,178
10.15
0.118
1,127
10.74
1.7
-0.58
New Hampshire
1,076
6.42
0.095
1,100
7.64
1.6
-1.21
New Jersey
7,563
7.58
0.030
7,623
8.16
0.8
-0.58
New Mexico
1,584
20.61
0.141
1,519
19.55
2.0
1.05
New York
17,482
13.03
0.040
17,938
12.57
0.7
0.46
North Carolina
6,397
12.97
0.059
6,301
12.22
0.9
0.75
North Dakota
614
14.38
0.210
642
12.31
1.6
2.07
Ohio
10,574
12.54
0.036
10,754
10.63
0.8
1.91*
Oklahoma
3,052
16.71
0.091
3,126
14.72
1.8
1.99
Oregon
2,776
12.42
0.080
2,915
11.22
1.8
1.21
Pennsylvania
11,536
11.13
0.033
12,141
10.39
0.8
0.84
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Small-Area Estimates of School-Age Children in Poverty: Interim Report I: Evaluation of 1993 County Estimates for Title I Allocations
Rhode Island
964
9.61
0.121
963
6.65
1.5
2.96*
South Carolina
3,368
15.37
0.101
3,446
16.98
1.8
-1.60
South Dakota
670
15.86
0.160
696
13.22
1.6
2.64
Tennessee
4,744
15.70
0.045
4,833
16.44
1.9
-2.73
Texas
16,580
18.10
0.044
16,886
17.06
1.0
1.04
Utah
1,694
11.36
0.104
1,683
8.20
1.4
3.16*
Vermont
541
9.86
0.145
556
7.91
1.6
1.94
Virginia
5,969
10.25
0.055
6.159
10.89
1.4
-0.65
Washington
4,741
10.92
0.058
4,729
9.62
1.6
1.30
West Virginia
1,755
19.66
0.134
1,799
15.73
1.9
3.93*
Wisconsin
4,754
10.70
0.054
4,694
8.37
1.4
2.32*
Wyoming
442
11.86
0.196
462
10.82
1.9
1.04
* Statistically significant difference from 0 at the 10 percent significance level.
a Standard errors of estimates from the 1990 census are calculated using 1980 census design factors.
SOURCE: Data from Bureau of the Census.
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Small-Area Estimates of School-Age Children in Poverty: Interim Report I: Evaluation of 1993 County Estimates for Title I Allocations
TABLE B-5 Poverty Rates for Related Children Aged 5–17 in 1989, by Selected Categories of Counties: 1990 Census and March CPS
Percent Poor Related Children Aged 5–17
County Category
1990 Census
March CPSa
Difference Between Rates
U.S. Total
17.0
18.0
1.0*
Metropolitan
Central
16.4
17.9
1.5*
Other
11.4
12.5
1.1
Nonmetropolitan
20.4
19.9
-0.5
Regionb
Northeast
14.3
15.5
1.2*
Northcentral
14.9
15.8
0.9*
South
20.5
21.3
0.8
West
16.2
17.3
1.1*
Population Size
Under 2,500
22.9
22.1
-0.8
2,500–4,999
22.2
14.6
-7.6
5,000–9,999
23.1
24.7
1.6
10,000–49,999
20.6
20.9
0.3
50,000–99,999
16.6
15.7
-0.9
100,000–499,999
14.7
15.7
1.0*
500,000–999,999
14.6
15.6
1.0
1,000,000 and over
19.1
21.5
2.4*
* Statistically significant difference from 0 at the 10 percent significance level.
a The CPS estimates are 3-year centered averages of data from the 1989, 1990, and 1991 March CPS (reported income in 1988, 1989, and 1990, with population controls derived from the 1980 census).
b The Census Bureau's regions are defined in Note c of Table 2-3.
SOURCE: Data from Bureau of the Census.
Representative terms from entire chapter:
household income
