Redesign of the Sample for the 

Current Population Survey 

Harland Shoemaker 

T

he Current Population Survey (CPS), sponsored by 

the Bureau of Labor Statistics (BLS) and conducted 

by the U.S. Census Bureau, is a monthly sample survey 

of households that produces reliable estimates of labor force 

characteristics of the working-age population. 

Since the inception of the survey in 1940, there have been 

various changes in the design of the CPS sample. The last 

major sample redesign of the CPS took place in the early 

1980s and involved a major effort to upgrade the quality and 

reliability of State-level data. Usually, a new sample is selected 

after each decennial census. The sample selected after the 

1990 census, introduced between April 1994 and July 1995 

and currently in use, is similar to the one designed in the 

early 1980s. Starting in April 2004, the 1990 sample began to 

be phased out through a series of changes that will be 

completed in July 2005. Again, the new sample is similar to 

the sample currently in use in terms of its size, although it 

does include some reallocation among the States to achieve 

the specified reliability requirements more efficiently. The 

principal reason for the redesign, however, is to maintain the 

efficiency of the sample design by updating the sampling 

frame with Census 2000 data. 

This article discusses the scope and nature of the changes 

to the 1990 sample design and their effect upon the survey 

methods and the estimates. It also describes how the new 

design is being phased in. 

Design objectives 

The CPS is a monthly probability sample based on a stratified 

sampling design that is intended to provide national and 

State estimates of labor force characteristics with a specified 

level of reliability. In the first stage of sampling, primary 

sampling units (PSUs), which correspond to substate areas, 

counties, or groups of counties that are geographically 

contiguous, are selected. These PSUs are then grouped into 

strata, and within each stratum, a single PSU is chosen for 

the sample. The probability of selection of this PSU, which 

represents the entire stratum from which it was selected, is 

proportional to its population as of the most recent decennial 

Harland Shoemaker is Chief, Current Population Survey Branch, 

Demographic Statistical Methods Division, U.S. Census Bureau. 

census. In the case of strata consisting of only one PSU, the 

PSU is chosen with certainty. 

The CPS sampling frame, which is based on a particular 

census, represents the actual population less efficiently over 

time. Replacing the 1990 sampling frame with one based on 

Census 2000 improves the efficiency of data collection by 

field representatives. In addition, the more recent and more 

accurate Census 2000 information was used to redefine the 

PSUs and to restratify them, incorporating variables that are 

more strongly related to the unemployment rate. Updated 

data from the BLS Quarterly Census of Employment and 

Wages, also known as the ES-202 program, also were used in 

the restratification as well. Finally, the updated Census 2000 

information was utilized to select a more efficient sample 

within the selected PSUs. 

The 2000 CPS sample design reliability requirements are 

the same as those for the 1990 design. The only difference is 

that, for the new design, the supplementary sample from the 

State Children's Health Insurance Program (SCHIP) survey, 

which was first used in the production of official CPS 

estimates in July 2001, was selected independently from the 

CPS sample. However, the data from both the CPS and the 

SCHIP survey will continue to be used to produce the official 

estimates each month. Reliability requirements tor the sample 

are expressed in terms of coefficients of variation (CVs). All 

references to CVs in this article include an assumed 

unemployment rate of 6 percent. 

The Nation. The required CV on the unemployment level for 

the Nation is 1.9 percent. The CV itself is based on the 

requirement that a difference of 0.2 percentage point in the 

unemployment rate for consecutive months be significant at 

the 90-percent confidence level. 

States and substate areas. For all States, except California 

and New York, the CV on the annual average unemployment 

level is no larger than 8 percent. In New York and California, 

this CV applies separately to the substate areas of New York 

City (Bronx, Kings, New York, Queens, and Richmond 

counties), the balance of New York State, the Los AngelesLong Beach-Glendale metropolitan division (Los Angeles 

County), and the balance of California. 
Phase-in of the new design 

The phase-in of the new CPS design will have two dimensions. 

The first will involve "continuing" areas—that is, those areas 

selected in both the old and new sample designs. The 

sampling frame in these areas began to be updated in April 

2004 and will continue through July 2005. Sample households 

selected from Census 2000 and building permits for new 

residential construction since Census 2000 will replace 

households selected from the 1990 census lists and from 

building permits for new residential construction issued since 

the 1990 census. Ninety percent of the entire sample is in 

these "continuing" areas. 

The second dimension of the phase-in involves changing 

the areas selected for sampling (or the sample PSUs). From 

August through November 2004, areas selected only for the 

new design gradually replaced those selected for the old 

design. 

The redesigned sample will conta in 824 of the 2,025 total 

PSUs, which cover the entire geographic area of the United 

States. In the 1990 design, 754 areas out of the 2,007 PSUs 

covering the Nation were in the sample. The numbers of 

PSUs in the respective samples are not directly comparable, 

however, because, in the 1990 design, some PSUs in 

New England and Hawaii were formed from minor civil 

divisions in addition to counties, while in the 2000 design, 

the PSUs are strictly county based. Also, the PSUs have 

been redefined to correspond to the new Core-Based 

Statistical Area (CBSA) definitions and to improve efficiency 

in field operations. 

In both designs, the sample is divided into eight 

approximately equal rotation groups. The two designs also 

use the same rotation pattern, which has been a part of the 

CPS since 1953. According to this pattern, a rotation group 

is interviewed for 4 consecutive months, temporarily leaves 

the sample for 8 months, and then returns for 4 more 

consecutive months before retiring permanently from the CPS 

(after a total of eight interviews). The SCHIP sample shares 

these features with the CPS. 

Continuing areas. From April 2004 through July 2005, the 

new CPS design is being phased in for continuing areas, one 

rotation group per month. The phase-in consists of gradually 

replacing the sample selected from the 1990 census-based 

frames with that selected from the Census 2000-based frames. 

The gradual aspect minimizes the impact on the continuity of 

the CPS data series. In the replacement process, sample sizes 

within individual areas may change, but the total monthly 

sample size for the survey will remain about the same. For 

each month from April through July 2004, an additional 

rotation group from the new sample was phased in. Since 

July 2004, half of the sample in continuing areas has been 

obtained from the new design. Due to the rotation pattern, 

this proportion will not change from July 2004 through March 

2005. Then, from April through July 2005, the old-design 

sample will again be replaced by the new-design sample, one 

rotation group per month. The phase-in of the new sample 

will be complete in July 2005. The phase-in pattern is depicted 

in table 1. 

New sample areas. In August 2004, the new sample areas 

began to replace the outgoing areas. The effects of this 

change are expected to be minor, because the sample in 

continuing areas accounts for about 90 percent of the 

national estimate. During the phase-in, only 10 percent of 

the sample at any one time involves transitional areas 

(incoming new areas or outgoing old areas). As shown in 

table 1, from August 2004 through November 2004, the new 

areas were introduced, two rotation groups per month. One 

of these replaced a retiring rotation group from the old design, 

and the other replaced a rotation group from the old design 

that would otherwise have returned to the sample for its fifth 

interview. For this reason, the normal rotation pattern in 

these areas was not preserved during the August through 

November 2004 period. The replacement of outgoing areas 

with new areas was completed in November 2004, 8 months 

before the phase-in of the new sample in continuing areas is 

scheduled to be completed. 

As the new areas are phased in, the base weights and 

weighting factors used in computing the estimates will be 

updated to account for the change in the areas that make up 

the sample. The factors include the noninterview adjustment 

and the first-stage estimation procedure. (See the section 

titled "Explanatory Notes and Estimates of Error" in this 

publication for descriptions of these procedures.) These 

updated factors will be introduced into the estimation process 

at the same rate as the new sample areas are intr oduced into 

the sample. 

Table 1. Rotation group redesign scheme 

Year and 

month 

Continuing areas 

(90 percent) 

Replacement areas 

(10 percent) 

Overall 

percent 

of 

households 

on 2000 

design 

Year and 

month New 

sample 

rotations 

Old 

sample 

rotations 

New 

sample 

rotations 

Old 

sample 

rotations 

Overall 

percent 

of 

households 

on 2000 

design 

2004: 

April 1 7 0 8 11 

May 2 6 0 8 23 

June 3 5 0 8 34 

July 4 4 0 8 45 

August 4 4 2 6 48 

September.... 4 4 4 4 50 

October 4 4 6 2 53 

November.... 4 4 8 0 55 

December.... 4 4 8 0 55 

2005: 

January 4 4 8 0 55 

February 4 4 8 0 55 

March 4 4 8 0 55 

April 5 3 8 0 66 

May 6 2 8 0 78 

June 7 1 8 0 89 

July 8 0 8 0 100 
Effects of the changeover 

Field operations. Because new areas will be phased into the 

sample and some old areas will be dropped, increased hiring 

and training of field representatives, as well as some 

dismissals, will be required during this period. To minimize 

the possible influence of inexperienced field representatives 

on the CPS program, trainees will have half the normal 

assignments the first month they interview (July 2004). In 

addition, all of these interviews will be personal visits, and 

the initial interview results will not be used for estimates. 

During their first month of interviewing, new field 

representatives also will have 3 days of classroom study, 20 

hours of self-study, and 2 days of on-the-job training with a 

senior field representative. Their workload will increase by 

one rotation group per month in each of the 4 months after 

July 2004; this additional sample will be included in the official 

estimates. Beginning in November 2004, all data collected 

by these new field representatives will be incorporated into 

the official estimates. 

Efforts also will be made to maintain the experienced staff 

of field representatives who will be affected by the phaseout 

of certain sample areas, by offering them work in adjacent, 

continuing areas or on other programs. In any event, they 

will be notified in advance of the phaseout of their sample 

areas. Their work during this period will be closely monitored 

to avoid any adverse impact that their anticipation of 

termination may have on the data. 

Continuity of data series. Three features of the new CPS 

design have the potential to affect published estimates: 

(1) the temporary disruption of the rotation pattern from 

August 2004 through June 2005 for a comparatively small 

portion of the overall sample, (2) the change in sample areas, 

and (3) the introduction of the new CBSAs (formerly called 

metropolitan areas). 

Labor force estimates in the CPS are affected by changes 

in the rotation pattern because of the relationship between 

the number of times a rotation group has been interviewed 

and the expected value of the estimate obtained from the 

group. Normally, the eight rotation groups contributing to 

the published estimate are evenly distributed in terms of the 

number of times they have been in the sample (that is, one 

rotation group in the sample for the first time, one for the 

second time,..., and one for the eighth time). However, when 

the new areas are phased in at the rate of two rotation groups 

per month, this even distribution by time spent in the sample 

is lost. For example, in August 2004, two rotation groups in 

new PSUs were in the sample for the first time and none for 

the fifth time. Because published CPS estimates represent 

averages of the eight rotation group estimates, a change in 

the distribution of the sample rotation groups' number of 

months in sample may result in a dmerence m the overall 

estimates for those months, even though the actual value of 

the characteristic being estimated may not have changed at 

all. Conversely, this effect may mask real changes in the 

estimates. In either case, the disruption to the rotation pattern 

is expected to have a negligible effect on the published 

estimates. 

In addition to the temporary phase-in effect just discussed, 

the new design may produce some small permanent 

differences, particularly in the State estimates. These 

differences are the result of restratifying the PSUs within 

each State for the 2000 design, selecting a different set of 

sample PSUs to represent each State, and adjusting certain 

weighting factors to reflect this change. The differences may 

appear to be discontinuities in the data series, but in fact are 

the result of variability arising from selecting only a sample 

of PSUs to represent entire States. 

The introduction of new sample areas also has an effect 

on data series, the upshot of which is that there will be slightly 

less overlap of the sample used to produce estimates of 

year-to-year change between 2004 and 2005. However, the 

increase in variability due to the smaller overlap is expected 

to be negligible, because the proportion of overlap in the 

samples will drop to a minimum of 45 percent from the usual 

50 percent. 

Finally, changes in the definitions of metropolitan areas 

(now called Core-Based Statistical Areas) could affect the 

CPS estimates of metropolitan populations at both the State 

and national levels. However, preliminary research has shown 

that the national-level estimate" of ihe metropolitan and 

nonmetropolitan populations should not change appreciably 

because of the new sample design.