SOURCE AND ACCURACY OF ESTIMATES



REDESIGN OF THE CURRENT POPULATION SURVEY/Housing Vacancies and Homeownership

(CPS/HVS)



     Major changes related to the Current Population Survey/Housing

Vacancy Survey (CPS/HVS) are effective beginning with the first quarter

1994 data.  First, a new weighting procedure was implemented based on the

1990 decennial census.  The 1990-based weighting produces, on average,

estimates of the total housing inventory that are about 0.1 percent lower

than the 1980-based weighting.  Generally, the vacancy rates are only

minimally affected, while the homeownership rate is about one-half of a

percentage point lower with the new weighting procedures.  

     A second change is that the CPS/HVS has become a totally

computerized survey with the implementation of the Computer Assisted

Survey Information Collection (CASIC).  The CASIC tools consist of state

-of-the-art computer-assisted modules for data collection and processing. 

Although the concepts, definitions, and questionnaire items remain the

same, the shift to CASIC may affect vacancy rates and homeownership

rates.  We are unable to determine the quantitative effects of the use of

CASIC on the vacancy and homeownership rates.  Data users should use

caution when comparing data for 1994 and later with earlier data.





SOURCE OF DATA 

 

     The estimates presented in this report are based on data obtained

from two surveys conducted by the Bureau of the Census. Data concerning

vacancy rates and tenure of occupied housing units are from the monthly

sample of the Current Population Survey (CPS).  Characteristics of

occupied housing units in table 3 are from the American Housing Survey

(AHS). 





CPS AND AHS DESIGNS 

 

     Since the inception of the CPS in 1940, the sample has been

redesigned several times to upgrade the quality and reliability of the

data and to meet changing data needs.  Beginning in April 1994, the

current design was phased in through a series of changes that were

completed in July 1995.  



     The CPS/HVS sample is selected from a frame based on the 1990 census

and is spread over 754 sample areas, which represent 2,007 geographic

areas in the United States.  Due to the new sample drawn from the 1990

census, metropolitan/nonmetropolitan data published in 1995 and later are

not directly comparable to data for 1994 and earlier.



     Beginning in the first quarter 1986, vacant seasonal mobile homes

were included in the count of vacant seasonal units.  This change

resulted in a 12 percent increase in the number of vacant seasonal

housing units.  

     Of the 60,000 housing units both occupied and vacant contained in

the CPS sample, approximately 51,000 are eligible for interview each

month; of this number, 3,200 occupied units, on the average, are visited

but interviews are not obtained because occupants are not found at home

after repeated calls or are unavailable for some other reason.  In

addition to the 51,000, there are also about 9,000 sample units in an

average month which are visited but are found to be vacant or otherwise

not to be interviewed.  About half of the 9,000 are vacant and

interviewed for HVS.   

     The CPS estimation procedure for occupied units involves the

inflation of the weighted sample results to independent estimates of the

total civilian noninstitutional population of the United States by age,

race, sex, and Hispanic/non-Hispanic categories. These independent

estimates are based on statistics from the decennial censuses of

population; statistics on births, deaths, immigration, and emigration;

and statistics on the strength of the Armed Forces. 

     The HVS estimation procedure for vacant units is similar to that

used for occupied units.  Weighted sample results are adjusted at the

state level using 1990 census vacant counts.  A second adjustment

inflates these results based on the CPS coverage of occupied units by

geographic areas. 

     Data shown in all tables (except table 3) on vacancy rates and

tenure of occupied units for the second quarter 1997 are from the CPS/HVS

and are averaged for the 3 months, April, May, and June 1997.  The data

concerning the distribution of characteristics for occupied housing

units, shown in table 3, are obtained primarily from the AHS national

sample.  Distributions of characteristics of occupied housing units from

the AHS estimates are applied to CPS current housing inventory

independent estimates to obtain the characteristics of occupied housing

units used in this report.  The Survey of Construction (SOC) and the

Consumer Price Index also are used to improve estimates of the rent

distribution.  

     The 1993 AHS sample is spread over 394 sample areas comprising 878

counties and independent cities with coverage in each of the 50 States

and the District of Columbia.  Of the 56,700 housing units both occupied

and vacant contained in the AHS sample, 51,100 were interviewed and 2,300

were classified as "noninterview" for various reasons.  In addition,

3,300 units were visited but were not eligible to be interviewed for the

purposes of AHS. 

     The AHS estimation procedure for occupied units is a three-stage

ratio estimation process.  The first-stage adjustment is made by region,

tenure, metropolitan area status, and urban or rural status using data

from the 1980 census.  The second stage makes adjustments to independent

estimates of new construction obtained from the Survey of Construction

which includes a survey of housing starts and completions conducted

monthly by the Bureau of the Census.  The third stage adjusts to

independent estimates of the total housing inventory based on 1990 census

data.  Then, the occupied portion and the vacant portion of the

independent control are allocated based on data from the CPS/HVS. A more

detailed description of the AHS sample design and estimation procedure

can be found in the AHS series H-150 report for 1993.  





COMPARABILITY WITH CENSUS OF HOUSING DATA 



     Most of the concepts and definitions are the same for items that

appear in both the 1980 and 1990 censuses and the Housing Vacancies and Homeownership. 

However, there is one minor difference in the housing unit definition

between the CPS/HVS and the 1980 and 1990 decennial censuses.  The

difference is that, in the CPS/HVS prior to 1983, living arrangements

containing five or more persons, not related to the person in charge,

were classified as group quarters; for the 1980 and 1990 census, the

requirement was raised to nine or more persons not related to the person

in charge.  There were some differences in what has been counted as

housing units between the earlier censuses and the CPS/HVS.  Descriptions

of the differences between earlier censuses and the HVS appear in the

1985 and earlier reports of this series. 

     Prior to the first quarter 1990, there were significant differences

between the CPS/HVS and the decennial censuses.  The 1980 and 1990

decennial censuses included vacant mobile homes as housing units, whereas

prior to 1986 the CPS/HVS did not.  However, beginning in the first

quarter 1986, vacant seasonal mobile homes were counted as housing units

in the CPS/HVS.  In addition, year-round vacant mobile homes were counted

as housing units, beginning in the first quarter 1990 in the CPS/HVS. 

Another difference in the housing unit definition between the CPS/HVS

(prior to 1986) and the 1980 and 1990 censuses was that the CPS/HVS

required units to be separate living quarters and have direct access or

have complete kitchen facilities.  For the 1980 and 1990 decennial

censuses, the complete kitchen facilities alternative was dropped with

direct access required of all units.  However, beginning in 1990, the

CPS/HVS requirement for complete kitchen facilities was dropped with

direct access required of all units.  Thus, the earlier definitional

differences were eliminated.  

     In addition, there are differences between the methodologies used to

collect data for the CPS/HVS and the censuses.  These differences include

interviewing procedures, staff experience and training; differences in

processing procedures and sample designs; the sampling variability

associated with the CPS/HVS and the sample data from the census; and the

non-sampling errors associated with the CPS/HVS and census data. 

     Research has shown that the CPS/HVS and the 1990 census produced

significant differences for vacancy characteristics.  The rental vacancy

rate from the April 1990 census was 8.5 percent, whereas, the CPS/HVS

reported the rental vacancy rate of 7.2 percent for the first half of

1990.  The April 1990 census had a homeowner vacancy rate of 2.1 percent,

while the CPS/HVS had a vacancy rate of approximately 1.7 percent for the

first half of 1990.  For occupied housing, the April 1990 census produced

a homeownership rate of 64.2 percent, while for the first half of 1990

the CPS/HVS produced a rate of 63.9 percent. 

     These differences illustrate that, for these characteristics as well

as others, caution should be used when making comparisons between the

1990 census and the CPS/HVS. 





COMPARABILITY WITH EARLIER DATA 

 

     As stated earlier in this report, beginning with the first quarter

1994 new weighting procedures based on the 1990 decennial census were

implemented.  In addidition, the survey data collection procedures became

totally computerized.  Caution should be used when comparing current data

with unrevised data prior to 1994.

     In the fourth quarter 1989, new edit procedures were implemented in

the Current Population Survey/Housing Vacancies and Homeownership (CPS/HVS).  These

new procedures were used to allocate cases that would have been

classified as "not reported" under previous procedures.  

     In the first quarter 1990, year-round vacant mobile homes were

included for the first time as part of the year-round vacant count of

housing units.  This change was made to make the composition of the

housing unit inventory for the CPS/HVS similar to the decennial census

and other surveys, which count all mobile homes as housing units when

occupied or vacant (available for occupancy on the site).  Research has

shown that the inclusion of vacant mobile homes increased the vacancy

rate significantly in some cases.  We have revised 1989 data in this

report to reflect all changes.  Caution should be used when comparing

data from 1990 or later with unrevised data prior to 1990.

     In addition to the design and estimation changes mentioned in the

previous section, caution should be used in comparing data for 1980 and

beyond in this report with data from 1979 and earlier years. Starting

with the first quarter 1980, several changes were implemented in the

survey to improve the reliability of the data presented.  These included

adding a supplemental sample, refining the estimation procedures, and

changing the source of occupied characteristics from the Quarterly

Housing Survey to the AHS. 

     Although the above mentioned changes have resulted in more reliable

estimates, data for 1980 and later in this report are not completely

comparable to data for the fourth quarter 1979 and previous quarters, as

published in Housing Vacancies reports, series H-111, Nos. 1 to 79-Q4. 

Furthermore, unrevised data prior to 1990 is not completely comparable to

1990 data and beyond, due to the inclusion of year-round vacant mobile

homes, beginning in the first quarter 1990.  Thus, particular caution

should be observed in drawing conclusions about trends that extend from

before 1980 to 1980 and beyond, and also trends from before 1990 to 1990

and later.  For comparative purposes, 1979 data in this report have been

revised to incorporate all changes made in 1980, and 1989 data have been

revised to incorporate all changes made in 1990. Unrevised 1989 and 1979 

data are provided to show the magnitude of the various changes.  





VACANCY RATES FOR CHARACTERISTICS IN TABLE 3 

 

     Vacancy rates in table 3 are based in part on forecasts of occupied

housing units.  These forecasts are periodically revised to incorporate

more recent data and improved forecasting procedures.  Beginning in the

first quarter 1995, table 3 is based on data from the 1993 AHS.  

     For the occupied unit forecasts for the monthly rent categories, we

update the AHS data quarterly to reflect the rise in the cost of renting

through the use of the residential rent index, and the latest available

asking rent data for newly constructed rental units.  



CAUTION IN USING SEASONAL VACANT DATA  



     Analysis of seasonal vacant data prior to the first quarter 1987 has

shown that estimates for these characteristics have been underestimated

by approximately 28 percent.  The estimates beginning with the first

quarter 1987 are adjusted to reflect this.  





ACCURACY OF THE ESTIMATES 

 

     Since the CPS/HVS estimates are based on a sample, they may differ

somewhat from the figures that would have been obtained if a complete

census had been taken using the same questionnaires, instructions, and

enumerators.  There are two types of errors possible in an estimate based

on a sample survey:  sampling and non-sampling.  The accuracy of a survey

result depends on both types of errors, but the full extent of the non-

sampling error is unknown.  Consequently, particular care should be

exercised in the interpretation of figures based on a relatively small

number of cases or on small differences between estimates.  The standard

errors provided for the CPS/HVS estimates primarily indicate the

magnitude of the sampling error.  They also partially measure the effect

of some non-sampling errors in responses and enumeration; but do not

measure any systematic biases in the data.  (Bias is the difference

averaged over all possible samples, between the estimate and the desired

value.) 





NON-SAMPLING VARIABILITY 

 

     Nonsampling errors can be attributed to many sources, e.g.,

inability to obtain information about all cases in the sample,

definitional difficulties, differences in the interpretation of

questions, inability or unwillingness on the part of respondents to

provide correct information, inability to recall information, errors made

in collection such as recording or coding the data, errors made in

processing the data, errors made in estimating values for missing data,

and failure to represent all units with the sample (undercoverage). 

Undercoverage in the CPS/HVS results from missed housing units and

misclassifying housing units. Ratio estimation to independent controls,

as described previously, partially corrects for the bias due to survey

undercoverage.  However, biases exist in the estimates to the extent that

missed households have different characteristics than interviewed

households.  





SAMPLING VARIABILITY 

 

     The standard errors shown in the tables are primarily measures of

sampling variability, that is, of the variations that occurred by chance

because a sample rather than the entire population was surveyed.  The

sample estimate and its standard error enable one to construct confidence

intervals; ranges that would include the average results of all possible

samples with a known probability.  For example, if all possible samples

were selected, each of these being surveyed under essentially the same

general conditions and using the same sample design, and if an estimate

and its standard error were calculated from each sample, then

approximately 90-percent of the intervals from 1.6 standard errors below

the estimate to 1.6 standard errors above the estimate would include the

average result of all possible samples. 

     The average estimate derived from all possible samples is or is not

contained in any particular computed interval.  However, for a particular

sample, one can say with specified confidence that the average estimate

derived from all possible samples is included in the confidence interval. 

     Standard errors may also be used to perform hypothesis testing, a

procedure for distinguishing between population parameters using sample

estimates.  The most common types of hypotheses appearing in this report

are:  (1) the population parameters are identical, and, (2) the

population parameters are different.  An example of this would be

comparing the vacancy rate in MAs versus the vacancy rate outside MAs. 

Tests may be performed at various levels of significance, where a level

of significance is the probability of concluding that the characteristics

are different when, in fact, they are identical. 

     To perform the most common test, let x and y be sample estimates for

two characteristics of interest.  Let the standard error on the

difference x-y be SEDIFF.  If the ratio R = (x-y)/SEDIFF is between -1.6

and +1.6, no conclusion about the difference between the characteristics

is justified at the 0.10 level of significance.  If, on the other hand,

this ratio is smaller than -1.6 or larger than +1.6, the observed

difference is significant at the 0.10 level.  In this event, it is a

commonly accepted practice to say that the characteristics are different.

Of course, sometimes this conclusion will be wrong.  When the

characteristics are, in fact, the same, there is a 10 percent chance of

concluding that they are different.  All statements of comparison in the

text have passed a hypothesis test at the 0.10 level of significance or

better.  This means that, for most differences cited in the text, the

estimated difference between characteristics is greater than 1.6 times

the standard error of the difference. 

     Comparisons of characteristics of the vacancies for the first

quarter 1990, (which include year-round vacant mobile homes as part of

the year-round vacant housing inventory for the first time) with previous

unrevised quarters reveal significant differences in some cases.  Thus

caution should be used when comparing current data with unrevised data

prior to 1990.

 

ILLUSTRATION OF THE USE OF TABLES OF STANDARD ERRORS 

 

     Standard errors are used to: 1) measure the accuracy of the survey

estimates, and 2) draw inferences from the survey data.  For example,

Table 10 of this report shows that 2.6 percent of all housing units are

vacant and available for rent.  Table A-1 shows the standard error of

this estimate to be approximately 0.1 percentage points.  Consequently,

the 90-percent confidence interval as shown by these data is from 2.4 to

2.8; i.e., the interval 2.6 + (1.6 x 0.1) percentage points.  Thus, one

can say with about 90-percent confidence that the average rental vacancy

rate derived from all possible samples is included in this confidence

interval.  Statements about differences are made only when the 90-percent

confidence interval on the estimated difference does not include zero. 

The 90-percent confidence intervals are shown in the text for selected

items.  The standard errors for other figures in this report are given in

the tables. In addition to sampling error, the figures in this report,

both the estimates and their standard errors, are also subject to

rounding error.