<b><font size=+1>APPENDIX B.  SOURCE AND ACCURACY OF ESTIMATES
REDESIGN OF THE CURRENT POPULATION SURVEY/Housing Vacancies and Homeownership
(CPS/HVS)</font></b>

     Major changes related to the Current Population Survey/Housing Vacancy 
Survey (CPS/HVS) were 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.  Revised data are provided in the historical tables for 1993 to
show the effect of this change.  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 1994 and later data 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/Housing Vacancies and Homeownership (CPS/HVS).  Characteristics of occupied
housing units 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 1984, the current design was phased in 
through a series of changes that were completed in July 1985.  Prior to the 
redesign, sample cases were selected from the 1970 census frame. 

     The CPS/HVS sample is spread over 729 sample areas, which represent 1,973
geographic areas in the United States.  The metropolitan/nonmetropolitan data 
shown in this report reflect 1990 census definitions.  The CPS/HVS began a major
geographic redesign in April 1994.  The survey gradually replaced sample cases 
selected from the 1980 census over a 15-month phase-in period with new sample 
cases drawn from the 1990 census.  The sample was fully phased in by June 1995. 

    For the transitional period (1995), we have converted the remaining 1980 
sample cases to reflect 1990 metropolitan/nonmetropolitan definitions.  
For 1996 data and beyond, the data will reflect 1990 definitions.

     In 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 occupied and 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 occupied 
units, there are also about 9,000 sample units in an average month which are 
visited but are found vacant or otherwise not to be interviewed.  About half of
the 9,000 are vacant and interviewed for the 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 2) on vacancy rates and tenure of 
occupied units for 1999 are from the CPS and are averaged over the 12 months 
of the year.  The data concerning the distribution of characteristics for 
occupied housing units, shown in table 2, 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, 
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 CPS/HVS appear in the 1985 and earlier reports 
of this series. 

     Prior to 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 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 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 nonsampling 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 in 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 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 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 year-round vacant mobile homes 
increases the vacancy rate significantly in some cases.  All of the 1989 data 
in this report have been updated to include year-round vacant mobile homes.  
Caution should be used when comparing unrevised vacancy data prior to 1990 to 
data for later years.

     In addition to the above mentioned design and estimation changes, caution 
should be used in comparing data for 1980 and beyond in this report with data 
from 1979 and earlier years. Starting in 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 (QHS) 
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 1979 and previous years, as published in Housing Vacancies reports, 
series H-111.  Furthermore, unrevised data prior to 1990 are not completely 
comparable to 1990 data and beyond, due to the inclusion of year-round vacant 
mobile homes, beginning in 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.  

     The revised 1979 vacancy estimates are higher than the original 1979 
estimates.  The increase in vacancy rates was not the result of locating 
additional vacant units, but reflects the increase in sample size and 
refinements in the estimation procedure.  It is safe to assume that prior to 
the implementation of these new procedures (1955 through 1978) HVS produced
underestimates of vacant units.  Earlier reports in this series give more 
complete descriptions of the original CPS sample, the QHS sample, and 
estimation procedures. 


CAUTION IN USING VACANCY RATES FOR CHARACTERISTICS
IN TABLE 2 

     Vacancy rates in table 2 are based in part on forecasts of occupied 
housing units.  These forecasts are periodically revised to incorporate more 
recent data and improved forecasting procedures.  For 1999 and 1998 data shown 
on table 2, these forecasts are based on the 1995 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 1987 has shown that 
estimates for these characteristics have been underestimated by approximately 
28 percent.  The estimates beginning in 1987 are adjusted to reflect this.  
This revision has an effect on other categories (especially the percentage 
occupied) in addition to seasonal vacant units in the distributions shown in 
tables 7. 


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 nonsampling.  The accuracy of a survey result depends on both 
types of errors, but the full extent of the nonsampling 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 nonsampling 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.) 


NONSAMPLING 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 given in the following 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 
MA's versus the vacancy rate outside MA's.  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 vacancies for 1990 (which include year-
round vacant mobile homes as part of the year-round vacant inventory for the 
first time) with previous unrevised years reveal significant differences in 
some cases.  Thus caution should be used when comparing current data with 
previous 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 B-1 
of this report shows that the percent of for-rent units outside MAs for 1999 is 
estimated to be 2.2 percent.  
Table B-1 also 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 
2.0 to 2.4; i.e., the interval 2.2 +/- (1.6 x 0.1) percentage points.  Thus, one 
can say with about 90-percent confidence that the average percent of for-rent 
units 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. 
  

ILLUSTRATION OF THE COMPUTATION OF THE STANDARD ERROR OF A
DIFFERENCE
 
     Table B-1 shows the rental vacancy rate for units in the South is 10.3 
percent and 6.2 percent in the West.  Thus, the apparent difference between the 
two rates is 4.1 percent.  The standard error of 10.3 percent and the standard 
error of 6.2 percent are both 0.2 as shown in table B-1.  Therefore, the 
standard error of the estimated difference of 4.1 percent is about 0.3 percent.     

                                ----------------
                                |
                         0.3 = \|(0.2)2 + (0.2)2
                                 
      Consequently, the 90 percent confidence interval for the 4.1 difference is
from 3.6 to 4.6 percent; i.e., the interval 4.1 + (1.6 x 0.3) percentage points.
Thus, one can say with about 90 percent confidence that this interval includes 
the actual value that would have been obtained by averaging the results from all
possible samples of this type.  Thus, we can conclude with 90 percent 
confidence that the rental vacancy rate in the South is higher than the rate in 
the West.