REDESIGN OF THE CURRENT POPULATION SURVEY/Housing Vacancies and Homeownership
(CPS/HVS)

	Major changes related to the Current Population Survey/Housing Vacancies and Homeownership
(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 first quarter 1998 are from the CPS/HVS and are averaged for 
the 3 months, January, February, and March 1998. 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 1995 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 59,500 housing units both occupied and vacant contained in the 
AHS sample, 51,700 were interviewed and 4,200 were classified as "noninterview" for
various reasons.  In addition, 3,600 units were visited but were not eligible to be
interviewed for the purposes of AHS. A detailed description of the AHS sample design 
and estimation procedure can be found in the H-150 report for 1995.  

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 third quarter 1997, table 3 
is based on data from the 1995 AHS.  The 1997 data shown in table 3 have been
revised to reflect this change.

	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 2.5 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 2.3 to 2.7; i.e., the interval 2.5 + (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. 

Table A-1.  Standard Errors of Percentages in Tables 10 and 11:  First Quarter 1998


                                  Vacant for-rent units as a 
                                  percent of all housing units

                                                         Standard
Area                               Standard    Differ-   error on
                           Rate      error     ence\1   difference


United States..........       2.5        0.1                   0.1
  Inside MAs...........       2.7        0.1        0.1        0.1
  Outside MAs..........       2.1        0.1       -0.1        0.2

 Northeast.............       2.2        0.1                   0.2
 Midwest...............       2.2        0.1       -0.3        0.2
 South.................       2.8        0.1        0.1        0.1
 West..................       2.7        0.1        0.2        0.2



                                  Vacant for-sale-only units as a 
                                  percent of all housing units

                                                         Standard
Area                               Standard    Differ-   error on
                           Rate      error     ence\1   difference


United States..........       1.0        (z)                   0.1
  Inside MAs...........       1.0        (z)                   0.1
  Outside MAs..........       1.2        0.1       -0.1        0.1

 Northeast.............       1.0        0.1                   0.1
 Midwest...............       0.8        0.1                   0.1
 South.................       1.3        0.1        0.1        0.1
 West..................       0.8        0.1       -0.2        0.1

Z Less than 0.05.  - Represents zero.
\1 Represents difference in first quarter 1998 and first quarter 1997 rates.