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 are from the Quarterly Household Survey (QHS).

The Current Population Survey sample is spread over 449 areas comprising 863 counties and independent cities with coverage in each of the 50 States and the District of Columbia. Approximately 50,000 occupied households are designated for interview each month; of this number 2,250 occupied units, on the average, are visited but interviews are not obtained because the occupants are not found at home after repeated calls or are unavailable for some other reason. In addition to the 50,000, there are also about 8,500 sample units in an average month which are visited but are found to be vacant or otherwise not to be interviewed.1 Data on vacancy rates and tenure of occupied units for the fourth quarter` 1971 are averages over the 3 months, October, November, and December.

The data concerning characteristics of occupied housing units are from the Quarterly Household Survey (QHS). This sample is spread over 235 areas, comprising 484 counties and independent cities. The figures for this report are based on the average of four quarterly estimates produced from interviews conducted in January, April, July, and October of 1970 and 1971. The estimates for the four quarters are each based on a one-third sample of the QHS households. Estimates for a full QHS sample are based on interviews conducted in approximately 11,500 households. These households come from larger sample of 16,000 occupied housing units. The smaller sample is chosen by identifying the households with annual family income less than $5,000 and interviewing a sample of one-half of these. The units with income less than $5,000 retained in the sample are given twice the weight of the other units in the sample to compensate for this subsampling. In addition to the 11,500 occupied housing units interviewed, about 1,000 occupied units are visited but interviews are not

a

1A more complete description of the Current Population Survey sample is given in Current Population Reports, Series P-23, No. 22, June 1967.

obtained because the occupants are not found at home after repeated calls or are unavailable for some other reason. There are also 2,000 units in the QHS sample visited but found to be vacant or otherwise not to be enumerated. Data for these vacant units are not included in the figures for this report.

The estimating procedures used for the QHS involve the inflation of the weighted sample results to independent estimates of the number of owner-occupied and renter-occupied housing units by urban and rural residence based on data from the Current Population Survey (CPS). These independent estimates have a lower sampling error due to the larger sample size and because other current independent estimates of the population of the United States are employed in the CPS estimation procedure.

Reliability of the estimates.--Since the estimates are based on a sample, they may differ somewhat from figures obtained if a complete census had been taken using the same schedules, instructions, and enumerators. As in any survey work, the results are subject to errors of response and of reporting as well as being subject to sampling variability.

The standard error is primarily a measure of sampling variability, that is, of the variations that occur by chance because a sample rather than the whole of the population is surveyed. As calculated for this report, the standard error also partially measures the effect of response and enumeration errors but does not measure any systematic biases in the data. The chances are about 68 out of 100 that an estimate from the sample would differ from a complete census figure by less than the standard error. The chances are about 95 out of 100 that the difference would be less than twice the standard error.

Comparison of characteristics of the vacancies for the fourth quarter 1971 with previous quarters (Series H-111, Nos. 1 to 66) reveals that many of the differences are small. A small difference,

particularly when it is based on a small subclass of vacant housing units, should be interpreted with care. As can be seen from table A, the sampling variability in such cases may be large relative to the difference.

Illustration of the use of table of standard errors.-Table 1 of this report shows that the rental vacancy rate for the fourth quarter of 1971 is estimated to be 5.3 percent. Table A shows the

standard error of this estimate to be approximately 0.2 percent.

The chances are 68 out of 100 that the estimated 5.3 percent would be within 0.2 percentage points; of a complete census figure, and chances are 95 out of 100 that the estimate would be within 0.4 percentage points of a census figure; i.e., this 95 percent confidence interval would be from 4.9 to 5.7 percent.

U.S. DEPARTMENT OF COMMERCE Bureau of the Census

1970 Census Users' Guide

Part 1 and Part 2

The 1970 Census Users' Guide is a two-part publication designed to furnish most of the information data users will need for effective access and use of 1970 census data products.

Part 1 of the Guide (standard, paper-bound publication) includes the text and three appendixes. The text covers such subjects as the collection and processing of 1970 data, data delivery media (computer tapes, microfilm, and printed materials), maps and information on how to obtain census materials. The appendixes are:

1970 Census Users' Dictionary - defines concepts associated with population and housing tabulations and geographic areas relevant to the collection and publication of data.

Comparison of Printed Reports and Summary Tapes summarizes and compares the contents of the reports and tapes.

Glossary-defines many terms used in connection with collecting, processing, and publishing census data, and lists many abbreviations relevant to the census.

Part 2 of the Guide (prepunched for 3-ring binder) contains appendixes specifically related to the use of census summary tapes and the Address Coding Guide.

Technical Conventions and Character Set - present information on the physical characteristics, format, and languages associated with tapes released by the Bureau.

1st-4th Count Technical Documentation - describes the arrangement of geographic codes and census data on the first four series of summary tapes.

Address Coding Guide Technical Documentation - furnishes information on the format and content of ACG's.

Many data users will find both Parts 1 and 2 of great value. Part 1, with its comprehensive coverage of the decennial census program, data products, and related services, is an important instructional and reference tool. Part 2, concerned exclusively with computer tape products, is designed particularly for those who plan to obtain tapes or who want complete information on the data content of the summary tapes.