Issue Brief

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Who Gets Missed in the Census?

Evaluating the Census Net and Differential Undercounts

By Eugene Ericksen, Temple University*

The goal of the census is to count each and every American once, at the correct residence. Since

the time of Thomas Jefferson, the Census Bureau has been painfully aware that not all are

counted. In recent censuses, it has also learned that many are counted twice, or at the wrong

location.

The net undercount is the difference between the number of people missed and the number of

people counted twice, counted in the wrong place or fabricated by an enumerator. Since 1940,

the Census Bureau calculated the net undercount by comparing its counted population to a

demographic estimate that is based on the Bureau's tabulations of birth, death, and migration

statistics. In 1990, more specific information about the undercount came from the Bureauís

follow-up survey to the Census.

Between 1940 and 1980 the net undercount declined substantially, from 5.4 to 1.2 percent, but

rose in 1990 to 1.8 percent.

0

1

2

3

4

5

6

Year 

1940195019601970

1980

1990

Percent

Percent Net

Undercount

Net Undercount

Figure 1.

 *

 Eugene P. Ericksen is a Full Professor at Temple University in Philadelphia, PA. During the 1990 Census, Dr.

Ericksen served as the Co-Chair of the Special Advisory Panel to the Secretary of the United States Department of

Commerce.
Figure 1 documents substantial improvement in the Bureau's ability to count the population over

time. However, focusing on the net undercount is a deceptive measure of the censusí accuracy

and reliability in several ways.

If the number of people missed and the number of people counted twice were equally high, the

errors could offset each other and the net undercount would be close to zero. If, for example, 10

percent of Americans were omitted but 9 percent were counted twice, the net undercount would

be just 1 percent, even though the rate of overall error would be much higher. However, the

rates of undercount differ among geographic areas and between whites and non-whites.

Figure 2 shows, the difference between undercount rates for African-Americans and non

African-Americans.1

 While the undercount for both groups declined between 1940 and 1980, and

rose slightly in 1990, the rates of undercount for African-Americans remained high. In 1980,

when the net undercount was 1.2 percent, the Black undercount was 4.5 percent, and the nonBlack undercount was only 0.8 percent. This racial bias in the census undercount is known as

the differential undercount.

Percent Undercount by Race

0

2

4

6

8

10

1940 1950 1960 1970 1980 1990

Year

Percent

African American

Non African American

Figure 2.

Figure 3 maps the difference in undercount rates between African Americans and all others from

1940 to 1990. In 1980, the differential undercount was (4.5 - 0.8 =) 3.7 percent, and in 1990 it

rose to 4.4 percent.

Differential Undercount

0

2

4

6

8

1940 1950 1960 1970 1980 1990

Year

Percent

Difference

Figure 3.

 1

 The Census Bureauís series of undercount estimates from 1940 to 1980 do not show the undercount for other

ethnic or racial groups.
Although the overall rates of net undercount have declined consistently, the differential has not.

It has varied within a narrow range, from a low of 3.4 percent in 1940 to a high of 4.4 percent in

1990. Based upon simple observation of the trend lines ñ net undercount decreasing and the

differential undercount increasing ñ we expect that Census 2000 will have a differential

undercount of about 4 percent.

As a theoretical matter, an overall net undercount of zero could mask a differential undercount.

To see this, let us assume that African Americans were one-eighth of the population and that

their net undercount was 3.5 percent. 2 If the net undercount of non-African Americans was -0.5

percent, a small overcount, then the net undercount for the nation would be:

3.5 percent * 1/8 - 0.5 percent * 7/8 = 0.

The differential undercount would be 4 percentage points (3.5 + .5). It would be consistent with

the differential undercount obtained by the Census Bureau during the last six decennials.

Because Blacks and non-Blacks are still residentially segregated from each other, it is likely that

some areas would have higher undercount rates than others. Reducing the net undercount to a

figure near zero does not mean that we have improved the census. Reducing the differential

undercount to a suitably lower figure is better evidence of improvement.

Undercount Rates 1940-1990

1940 1950 1960 1970 1980 1990

Net Undercount 5.4 4.1 3.1 2.7 1.2 1.8

Black

Undercount

8.4 7.5 6.6 6.5 4.5 5.7

Non-Black

Undercount

5 3.8 2.7 2.2 0.8 1.3

Differential 3.4 3.7 3.9 4.3 3.7 4.4

 2

 In 1990, Africans Americans comprised 12.0 percent of the population before adjustment and 12.5 percent after

adjustment.